Building on what BlankTimes said, I think the problem is ABA encompasses a load of different things. In particular, as I understand it, it's core is:
- Breaking down skills into achievable components (which is very useful and I think most people would be hard pushed to criticise - it's very much like what a physiotherapist does teaching somebody to walk, but for social/communication skills); and
- Rewarding for "positive" behaviour and "withholding" rewards for "negative" behaviour.
The second one is the controversial bit to my mind, and can become like doing tricks for treats. There are even cases in the US of institutions using electric shocks which obviously brings the whole thing into disprepute. For a lot of people it has nasty connotations of "gay cure" therapies (and it's based on the same underlying research).
That's particularly the case using things like food or withholding attention as a motivator. If you use just positive feedback (looking happy, clapping, or whatever) or star charts then it's less different to typical parenting in my view, and less problematic (though maybe also less effective for some children!)
Then I think it makes a huge difference what you are using it to achieve. Something that will definitely help your child (e.g. toileting or stopping self-injurious behaviours) I think it's hard to disagree with as it makes your child more independent and less vulnerable. If you use it to stop things for the parent's benefit rather than the child (non-injurious stimming is the obvious one) is I think very hard to support. Then there is loads inbetween where I'm really unsure of the ethics. We have been using it to try to help DD socialise more. On the one hand, she seems motivated to do this and I've read accounts from several autistic adults saying they were glad to be pushed a bit outside their comfort zone to build these skills. On the other hand, other autistic adults have said that being put through ABA made them feel like their true self was broken/flawed/not acceptable to their families, with all the obvious mental health repercussions that brings.
Personally we still use an ABA consultant, but we deliver everything ourselves so we have full control, try to do it only in natural/fun settings (nothing stuck at a table - just use the techniques in day to day life), and we try to think very hard about whether the targets are really for DD's benefit or ours, and only do the former.
It's hard as there are so few studies of the longer term psychological impacts or impacts on well-being. So it really feels like this generation of parents are flying a bit blind and we just have to read what we can and try to make the best judgement for our kids.