If I hear someone say "Legend X is fine and balanced", I also expect them to point out why. Because after all I have to provide arguments and points whenever I claim something being overpowered or underpowered. Equally should someone provide what makes them feel like something is fine. As long as that isn't given, there is no foothold to that claim.
Yes, Caustic is only good in certain areas. But there's still a difference between being good in an area, and being too good in an area. And I believe any legend with stationary abilities is well suited for buildings, tunnels or small places. But that's exactly what makes me think that caustic is straight up annoying. He just has to throw one gas trap in one small space and you're screwed. You can't see him, he can see you, you take continuous damage, you're slowed severely, your team mates can't reach you nor support you. But it's not about Caustic here, it's about Crypto.
And yes, even just being able to send him to places with the click of a button while in drone mode would be a sufficient change.