Despite the downer mood about her, I'm just trying to figure out what she Can do for now. Honestly, it's mostly a list of Can'ts. Almost all clever customizations I've attempted for her are outright failures for seemingly no good reason. Be aware of all these things as you use her, since there are no more hotfixes until after Tennocon (unless there's an emergency). For those in the future, this post is about Yareli's state after the Sisters of Parvos update and a couple hotfixes.
No method of incoming damage mitigation - be it from mods, arcanes, or even glitching her to use the dual stance for pistol+glaive and melee blocking that way - will make a difference while she's riding Merulina. Merulina will take damage regardless of any of that, as if Yareli is fresh and unmodded, just with its own 75% damage reduction applied.
You can have her hold the pistol + glaive dual stance while riding by equipping a pistol and glaive, then hold out your primary weapon when you cast Merulina. You'll automatically have your pistol out. Tap F to put it away and then F again to bring it back, and she will also bring out the glaive. You can passively melee block this way, and it will build the melee combo meter. Again though, it will not provide any protection for Merulina. You cannot melee attack during this and attempting to do so will dismount Merulina.
While riding Merulina, Yareli's shields will deplete from incoming damage but her health will remain intact afterward, except for some edge cases like Toxic Ancients where she loses some of her own health. Despite Yareli's shields being up and shield gating once they're down, Merulina will still take damage as if Yareli's health is exposed.
While riding, incoming damage still triggers effects that require it to function, such as Adaptation and Arcane Grace. These can be minorly useful since they'll already be active by the time Yareli is exposed, in case Yareli doesn't have the energy to recast Merulina.
Merulina itself cannot be modded. The only modding that affects it is Yareli's power strength. It innately has the bonus of Poppin' Vert: you can double jump by doing a flip or spin.
Merulina usually prevents Vacuum and Fetch from working while riding. It retains the default item acquisition distance, but sometimes pickups will reach Yareli and float at her midsection without being obtained until she dismounts Merulina.
Merulina gains Ventkid XP during use if you get trick scores while riding.
Speed modifiers of any kind won't boost Merulina's cruise or boost speed.
Ocucor fails to work at all while riding, and Catabolyst's reload grenade toss cannot be aimed properly. There are probably more inconsistent pistols than these, but I wanted to try these first and they both break.
Pistol aim will suffer a bit from riding as Yareli's bouncy riding movement will continue to move the origin of the weapon while your crosshair doesn't bounce. There's no sensible way to solve this, just be aware of it, especially with weapons that can self-stagger.
Primed Sure Footed will have no effect in preventing you from getting knocked off Merulina, from your own stagger or from enemy stagger. The only helpful thing is to use a mod like Cautious Shot to remove your own weapons' self stagger.
Many sources of knockdown or stagger are ignored while riding Merulina. Ancient and Scorpion grapples are ignored as well as laser-grid doors most of the time, but Heavy Gunner slams will dismount you.
Most "Press X" interactions are able to be done while riding. Yareli lowers herself a bit to be in range of the actions as you approach them. It is very difficult to give a key to towers in Disruption.
Yareli's X interactions will freeze her animations during and look sort of silly.
You cannot use operator mode while Yareli is riding. It will dismount Merulina first if you try. This prevents her from smoothly using the operator to traverse sections that are incompatible with riding.
Merulina blows up unexpectedly from small level details even while at low speed. This is similar to being launched from your K-Drive over what seems like simple terrain. Yareli doesn't get ragdolled from riding Merulina so it's less of an issue, and recasting is cheap, but you're still forced to spend energy and become exposed when nothing actually happened.
Yareli's height is lowered a bit automatically while riding when level obstacles would obstruct her movement, but she still reaches a height taller than a standing warframe during this, and gets stuck in many places. She can use the crouch/slide input for a brief spin that lowers her more, but it won't lower her continuously by being held, so she'll still be stuck. Many doors in older tilesets will halt her movement or rob most of her momentum from a brief scrape due to their low clearance. This is the opposite issue from Grendel, who often gets halted while rolling by small features that frames can normally walk over, such as the lower edges of doors.
Bonking into a ceiling from jumping too high while riding will not only take your momentum but often send you into reverse. Merulina can barely accelerate after this, so if you think you're close to landing on a nearby ledge, you'll often fail and fall.
Casting Sea Snares will project them outwards entirely horizontally. They will not follow your crosshair so you can't aim their placement. Have Yareli stand at the height you want them to float at before you cast.
Sea Snares will often leave miniature bubbles lingering in the level past their intended duration, but will do nothing, and won't prevent you from casting new ones up to the cap of 15.
Sea Snares will trap Liches, Sisters, Thralls, and Hounds, but not Demolysts of any kind. With special enemies like that, they won't even attempt to attach to them.
Sea Snares will seek out nearby enemies even if they're physically blocked from doing so. They'll slide along the environment and try to slip through, sometimes eventually ignoring obstacles to attach to enemies anyway.
Despite her passive for boosting crit chance on pistols, her signature pistol does not synergize well with it. Since the passive specifically modifies crit chance, you are discouraged from using any pistols that can't crit well.
Despite Riptide's size, it often won't drag in enemies that are partially behind ground-level short obstacles.
The level geometry matters when you're aiming where to cast Riptide. If the surface is at an angle, Riptide will stick out at that angle.
If an enemy is between you and the spot you're aiming at to cast Riptide, it will often fail to initiate a cast.
Subsume powers can't be cast while riding. Xata's Whisper stops providing a benefit while Yareli is on Merulina, though it stays active. Shock Trooper, Smite Infusion, and Fireball Frenzy DO still affect her pistol while riding. None of these affect her powers' damage or status procs. There is no damage type change, no damage amount change, and no status change.
Sea Snares and Riptide won't scale with the enemies' health, which means in higher level content the damage is negligible regardless of modding. They simply become interruptions after a certain level. Riptide scales to the enemy count affected by it, but this usually won't make much of a difference, especially for being her most costly power.
Aqua Blades is at least still a passive stagger aura after the damage becomes negligible. It cannot be affected by range.
Since we can't use any mods to support Merulina besides using more power strength for a larger health bar, Merulina will explode fairly quickly in high level content. Yareli will spend much of her time recasting Merulina to keep herself alive, which honestly isn't too difficult despite the issues. Consider using her other powers for crowd control to keep Merulina alive for longer, but keep in mind that casting 4 will prevent you from using any actions with actual lethality for the casting time.
Forcing +105% Additive Critical Chance through using Arcane Avenger + Cat's Eye does not allow her abilities to crit. Riptide gives yellow damage numbers on the final hit but does not actually crit.
Riding Merulina tends to prevent squad affinity sharing.
Cameras do not detect Yareli if she's riding.
Magus Drive has no effect on Merulina's speed.
The evasion buffs from the skills of the Sly Vulpaphyla do work while riding, but they must be triggered while on foot. You cannot trigger them while riding.
Yareli gains no energy when riding into a Zenurik bubble.
I'm editing this periodically as as I test more, and as more things come to mind.