Merulina honestly feels fine to control when strafing and moving without boosting. There's other issues and I'd like to echo the suggestions posed by some community members of reducing Yareli's size when using Merulina à la Titania, in order to make her style of momentum-based movement work in the cramped spaces of normal missions, but discounting the environment it still feels smooth and natural to move. Then you press shift and suddenly it all changes, hence my suggestion:
Change the A and D keys when boosting on Merulina to work the same as they do when not boosting, as "strafe" keys.
In practice I understand why it is the way it is now. for Warframes when you press A and D when not sprinting you strafe as you can still aim and shoot, so it would look awkward otherwise. Meanwhile for visual purposes when sprinting, your Warframe will physically turn to face the diagonal or side you sprint to with the A and D keys as you cannot shoot, and sprinting sideways like a crab would look a bit unusual. This makes sense and poses no issue as Warframes can immediately switch their movement direction in an instant which leaves sprint still feeling responsive and this distinction entirely visual.
For Merulina however, it isn't a purely visual decision. Regardless of whether you're moving forward or not, when not boosting on Merulina you are able to strafe side to side with the A and D keys. This is great, it feels incredibly responsive and gives you complete control over your positioning and works exactly how you'd intuitively expect to. When you sprint however, the A and D key turn Merulina which leads to some consequences that I would think are unintended. Pressing the A or D key while sprinting at any point even for an instant totally changes your heading away from your mouse pointer for some time even after letting go of the key, and it's very difficult to predict where you're actually going to end up once it corrects itself. That in and of itself is mildly annoying, but then when you factor in turning with the mouse it just becomes completely uncontrollable.
I feel how it was intended to work was that you could turn corners using the mouse OR the WASD keys, but instead it results in WASD being awkward as you can't see where you're going and even accidentally trying to use both methods in tandem as you would when not boosting (to make a sharper turn by pressing A and moving left or a wider bank by pressing D while turning left, but ultimately ending up in the same space, resulting in more control to the player - how it works when not boosting) instead you veer off in a totally unpredictable direction upon pressing the key while turning, only to then veer off in another totally unpredictable direction when it attempts to correct after releasing the key. It makes boosting around tight corners, something that should be incredibly fun and exciting, instead frustrating and clunky. You build up muscle memory of using the movement keys when not boosting and it results in massive frustration when you naturally go to press them when boosting.
The immediate precision and responsiveness of camera-based control requires supporting control schemes to match that precision and responsiveness. Please chang A and D while boosting to instead apply sideways movement without physically turning Merulina. This would prevent it shifting her direction drastically and uncontrollably and would allow it to match and mesh with the mouse controls.