Grendel feels close to being on par with other frames but I feel like he falls short in a few ways, mainly his lack of scaling damage, lack of clarity for his nourish buffs, and how holding enemies in his stomach is not worth the energy drain.
While Grendel isn't really meant to be a damage dealing frame like Saryn or Mesa, regurgitate at this point feels less like a damage ability using enemies as ammo and more like a way to cycle through the enemies in his gut. While this functionality is pretty useful I think that the ability should be able to do some decent damage. Pulverize is in an even worst spot, being more of a gimmick that any decent way to deal with enemy crowds. I think the solution to both of these issues is somewhat obvious, let both of these abilities scale based on the level of the enemies used for the abilities, similar to how frames like Nyx, Octavia, and the newly reworked Vauban naturally get stronger as their enemies do since their abilities somehow use their enemy's stats to calculate damage (Nyx just uses an enemy, Octavia's mallet deals more damage the more damage it takes so more damaging enemies get more damage thrown back at them, Vauban's new 3 deals more damage the stronger the enemies it targets are). I believe that Regurgitate should take a percentage of the health of the enemy getting shot out and add it to it's currently existing base damage, while Pulverize can take a smaller percentage of all currently swallowed enemies' health and add it to its damage. Not only would this make regurgitate a decent fireball-type ability, especially if you use heavier enemies as ammunition, these buffs would also make it more worthwhile to eat a large amount of enemies before pulverizing, since the more enemies inside Grendel's gut the more enemies it has to scale off of. Maybe this scaling damage could be the corrosive or gas damage many people are suggesting be added to Pulverize could be this scaling damage, while the base damage is impact.
For Nourish, the fix is much simpler, I believe it's main issue is just one of clarity and convenience. Adding a UI element that is easier to see (The ability icon changing is nice, but it is small and gets covered by the duration timer) to indicate the buff nourish will give could make the ability a whole lot easier to manage on its own. However, along with that new UI, a possible change could be to add a "tap to change hold to cast" system like Ivara or Wisp. The UI could show all 3 buffs (maybe even colour-code them) along with a number indicating how many enemies of that buff are currently stored in the gut, maybe even fade out the buffs that have no fuel and let you chose the buff by tapping 2 and cast it by holding it. Though to be honest that system may make things more clunky so just the UI without the functionality might be best.
For passive, I think it should give more than just armor, since it's technically a channeled ability and those tend to be extremely powerful to make up for the disabling of most sources of energy regeneration (Exalted weapons, Equinox's 3 and 4, Hildryn's 4, etc). The armor bonus should stay, but I think (re)adding an immunity to knockdowns (or maybe even all statuses?) as well as maybe a small health regen (aura) per enemy as well as perhaps a lowering of the drain overall. The immunity to knockdowns (and maybe all statuses) idea mainly came from the fact that Grind Hard Squad's video on Grendel mentioned it but it doesn't seem to be a part of the passive (I think it's bugged and I already mentioned it in the bug report forum), while the health regen idea comes from the fact that many others have said that the health recovery from Nourish is too low, so instead of just increasing that this passive regen could help supplement his recovery. The regen and CC immunity would make him similar to Oberon, which makes sense since both seem to fill to role of a support-tank hybrid type frame, and if his regen was an aura, similar to Chroma's buffs, it would play even more into Grendel's supportive nature, but just the regen itself (per enemy of course) as well as the CC immunity and the pre-existing armour would make it much more worthwhile to hold onto enemies for longer. If the idea of the health regen aura is too similar to Oberon (tho I think it's somewhat unique, allies only need to be near Oberon when he casts renewal to get it's healing while my proposed Gluttony healing would only apply when allies are close, like a mobile Garuda blood fountain), then a simple cut to the cost along with the CC immunity and the armor would work well.
(If the CC immunity was moved to the Masseter heavy attack passive then it should move back to Grendel and the Masseter should have a different passive with Grendel, like maybe let it make enemies drop health and energy orbs like the Broken staff, but only on heavy attacks)
TL;DR I believe Regurgitate and Pulverize should scale off of enemy HP, Nourish needs a UI element and maybe a way to switch the queued up buff, and Gluttony should have CC immunity and they should either also give it a small health regen (maybe an aura for very close team mates) or reduce its energy drain