Into the Breach had everything that I love in video-games: Mechs, giant monsters, turn tactics, TRUE tactics (your moves matter), innovative gameplay... and from the FTL creators, the very best rogue-like in my opinion. But then... when you play it... It seems everybody has the same reaction; It's good, and that's it. It feels something is missing, and now, maybe I know why:
In the beginning you just lack options. Having mechs are cool and the decisions feel critical from the start, but... they don't do much. You just really get options later on, but then the game, "ok, that's it. Here's a very hard mission, good luck."
You know when you're in FTL, or many other games, and you see your arsenal proudly? It feels ITB ends one step too short. Now you have the tools but... you don't really feel happy about'em. They don't chain into crazy combos, everything, although a little better, it's still very muted.
Which is also how I feel about the game feel. Everything is just so lifeless. With mechs, I want to go forward and do awesome things, but everyone is so tamed. The sound effects, although competent, don't have the punch. The graphics are nice, but just not enough. FTL is also not very a beautiful game, but everything feels much more "punchy"; Firing lasers, shooting missiles, getting shot... It feels intense. But in ITB the monsters just get bored after a while and go away.
And that's all a shame. I remember when it was released, everybody expected to receive FTL treatment; free DLC and adjustments. There's an awesome game in ITB, it's just one step too short. But then again... it just didn't. ITB was left unchanged. "Well, that's it".
Therefore, in short, great mechanics, but lukewarm execution. If you like tactics game you'll like it, but just like it.