One of the biggest problems is that we will no longer be able to 'port' pay.
So I'm on UPS3. If I fancied a change of school, they could offer me MPS (or whatever they thought I'd accept really); which effectively means that my current school is stuck with me for the next 26 years.
I'm quite happy there fwiw, but it will reduce mobility generally, & drive down pay overall, eg. for the teacher who re-locates for family reasons & finds themselves only able to attract the pay they had 5 years before. This will also depress employment prospects for new teachers - there'll be a pool of more experienced applicants available at the same price.
In theory, a HOD deciding on timetabling for next year might feel inclined to give a team member with a pay review coming up a nice over-achieving top set or a tiny & well-supported bottom set, whilst giving someone like me, as an old lag, a lower/middle ability group who are less likely to hit target. This might not match our respective teaching strengths - tail wagging the dog to students' detriment.
A suitably Machiavellian HOD who wanted rid of a teacher in their department might even deliberately give them teaching groups who are unlikely to hit target, thereby denying pay progression or triggering capability procedures in the case of an experienced teacher.
It also allows for some schools (chain Academies, anecdotally) to employ a crop of NQTs, deny them progression & then replace most of them annually - the good ones will go elsewhere, the rubbish ones will hang in there on NQT pay - creating a situation where the staff is mainly transient & overwhelmingly young & inexperienced. Not that our young, inexperienced staff aren't brilliant & shouldn't be valued - but these schools aren't ever going to establish the stability of having experienced staff with a long term commitment to the school. Schools need both.
It will discourage the 'collegiate' system whereby if I've spent all summer writing awesome resources I email them to my Dept. as I go, & present them at our first meeting back. Why would I share my stuff if we're competing for an arbitrary bonus, which will go to whichever of us gets the best y11 results?
Finally - monitoring teachers' performance is nothing new. I've had two PM meetings each year with my HOD for as long as I remember. The extra powers being suddenly given to Heads to determine pay aren't designed to 'reward' good teachers; you'd have to be awfully naive to believe that.