I think you may possibly need the controller on if you are using the zero-hand-off stuff. But you shouldn't typically need that, for most clients at least.

From what I've seen the advice is not to use ZHO unless you really can't make your setup work without it. I suspect it is a bit of hack and I don't think they've even supported it on the AC APs.

Normal roaming is very fast for me, Skype calls don't drop on my iPhone for example. You do sometimes need to tweak the power settings though, some people have to turn down the power to make hand off work well, which is non intuitive. Basically if your wifi footprint between APs overlaps too much you don't get good roaming behaviour. I luckily get away with leaving the power on my APs on auto.

The overlap issue isn't a Unifiy specific issue.