can't say i've noticed any bugs on my kit, but it might be specific to certain devices.
however new firmwares aren't forced pushed on to devices, the user has full control as to when (or if) anything gets installed. their forums publish lots of what about gets changed in each release.
what did you price up out of interest? considering you've already got a modem i'm not sure what you'd need that would end up at £400. maybe you over specced. also, remember that you don't need everything to be ubiquiti. you could get an edgeouter to be your router (very low priced, but powerful, and gives you 4 ports for a switch), add a bigger switch and an AP.
as i said though, it depends on your specific site requirements, how many switch ports you'd need, how many APs needed to cover the whole site. whether you want/need PoE, etc etc