I had no freezing or achievement unlocking problems yet. And I did co-op local many times. Maybe it's just an online co-op thing?
Yes the controls and response is sluggish but it's a game that isn't meant to have high replay value. 2 playthroughs and you're done pretty much. Kinda like Xmen, Double Dragon, Final Fight.. they are just beat'em ups with not much to do afterwards..