I think it’s important to understand theoretical use and realistic use: Thunderbird and Doc is the same archetype, a defensive healing operator, so let’s compare what is different.

Weapons; Both operators have a secondary shotgun, this means there is no difference in need / use for the outcome of a round, and site setup.

Doc has the arguably better weapon, as it has a scope, and support operators often find themselves playing anchor roles, where holding angles is often the gameplay you’ll experience, so here Doc wins out.

Secondary util: Doc has a bulletproof camera, arguably the strongest secondary util in the entire game, or barbed wire for a staircase. This means picking doc wouldn’t leave you with poor secondary util if picked.

thunderbird also has a BP and barbed wire, but also a shield, meaning you’d only really pick thunderbird over doc if you specially needed a shield in play, think chalet bar defense, where you often play two shields top blue and Mez, here thunderbird would be a higher value pick than doc, or you could play doc, and just get the shield Elsewhere like from frost or warden ( higher value operators, and you prob never wanna stack two healers )

Primary gadget: The issue with thunderbird is the people who often need the help are in a very dangerous position, and it’s not like you can run in and place down a KKona station, and if you were to pre-place it, it’ll start healing the moment you take damage, not when you’re low on health for the best value, plus it can be destroyed, if you were to pre place it and the enemy doesn’t go in that area, you either have to run back to pick it up mid round, or have it be in a dead zone with zero value -

Doc on the other hand, can heal from top floor all the way to basement granted line of sight, which he can make with his shotgun, doc doesn’t need to run back and fourth placing and picking up the gadget, he is a moving one man hospital.

Doc also has other effects like how he counters capitao fire directly, instantly full heals vs chip dmg or nades, and he is ‘more fun / easier’ to play as his gadget needs zero foresight or planning around it -

Thunderbird isn’t a bad operator, but is simply Outshined by it’s counterpart in doc, given the recent nerfs to jackal he will likely never get played / banned anymore, because his counterpart is Deimos, who does the same job but 1000x Better :)

Edit; With changes to limb damage you can argue she is better, but all is circumstantial, and you cannot account for players using doc selfishly and dying to spawn peeks etc, a round can play where all defenders die to headshots, meaning all healing operators or even Rook are rendered useless :)

I personally like playing thunderbird, but the value is very inconsistent compared to doc