The passive + warp + another biotic ability for detonation is all the adept needs. The rest of abilities are barely used, if at all. Sentinel has access to the same thing but rest of abilities are quite useful as well. Warp + Throw for explosions is more than enough. My main point was there is simply no reason to to pick adept over Sentinel who can fill the adept's place and also bring utilities of its own.
Engineer is unique, nothing can fill its place, not truly. Because no class has access to Incinerate, Drone and overload in the same time. Not to mention the turret. They lack real damage but they are the best class to disable defenses, which is basically all you face on insanity difficulty.
Soldier is just the tank of the game if nothing else. They can soak up damage which enables them to remain out of cover and shoot, only going back when reloading. So this allows them to deal highest weapon damage under enemy fire. Ashley was problematic because she was a solider who was not a tank, so she was less optimal. Soldiers could use some love too tbh.
It seems like you are talking more about ME3 and the mediocre combat system it has. Some of these problems would disappear if everything didn't generate a combo, the cooldown system and weapon balance wasn't half assed, etc. Sentinel isn't much better than Adept, it just has some extra damage reduction. So it is a little better, but you don't need the DR in the first place on insanity when everything dies to a couple combos. Sure, Adept should have gotten improved singularity via patch, but it wouldn't have really made it much more efficient since it can already plow through the game.
ME2 Adept is more compelling within its game since it has one of the few abilities that can actually CC protected units in the base game, and it is great at crowd control (which actually mattered in ME2, especially compared to ME1), and can self combo with warp bombs or physics combos. Biotics weren't win buttons, but they were powerful when used correctly. The fact that there was some actual nuance in power choice for each cooldown was a great thing.
Soldier hasn't been the tank of the game since ME1. It is in the running for weakest class in ME3.