It's the whole system that stinks though isn't it?
Teachers who aren't trained to teach kids with SEN, all too happily dumping things on a TA who often has no idea how to teach and no more training on SEN than the teacher.
Then, teachers abdicate responsibilities to the TA who is left to decide what a child needs and when. So, it is all too easy to say a child 'looks' ok to me.
So the whole system is often looking for guidance, and in fact stands or falls, on the quality of what is often the least qualified person in it.
This goes for OT therapy, SLT therapy, etc etc. All delivered by a TA who may, or may not, have any training.
It's little wonder that the research does show that TA support is often ineffective and frequently detrimental
But I would say that this is not because of the TA but because of the system the TA works in when s/he is expected to be all things to all people and where teachers struggle to manage even basic non-compliance issues in a classroom so are scared beyond reason by the thought of anyone being encouraged to do anythign differently.
The system lets teachers think it is someone else's problem, it let's schools see children with SEN as optional add ons (or even worse, an encumbrance), it leads statementing fights down one path - a fight for TA hours in m/stream or s/school.
None of it is working from top to bottom.