To be honest I have felt like the lockout was just slightly too short for successfully hitting an assist. But, the window isn't too short to be hit out of. The only problem I have with the short window is when I'm trying to hit my opponent, they block, I DEFINITELY HIT DURING THE TIME I WOULD MEATY THE ASSIST but game says no and they block and I don't meaty the assist and I get hit. I have no idea how it could be, or if it even should be fixed. I probably should just throw more.
I do however think assists aren't locked out for long enough after hitting them. Careless assist calls can and should be punished harder and the game just doesn't have the capacity to punish assists hard and safely in non-happy birthday situations. The closest thing I can think of is counter calling with H Brass since that takes off fairly huge chunks of assist lifebars, so with enough proper countercalls they're discouraged from using their assist. Maybe they could be locked out for longer depending on what you hit them with, or how much damage they take?