I hope you mean the AX86U Pro.
1. They offer piss poor support. I've ended up buying brand new products of theirs that have been EOL and replaced with a new revision of the same hardware, so you only get a couple of firmware updates before your hardware is more or less junk from a security point of view.
2. They have so many hardware revisions of their popular models that you don't really know what you'll get, unless you go to a shop and see the specific revision, so you can end up with five year old hardware that's been sitting on a shelf, which means zero support.
3. They've already been been fined by the FCC multiple times, this was a minor thing that also Asus got in trouble for (PDF link)
https://transition.fcc.gov/Daily_Releases/Daily_Business/2016/db0801/DOC-340564A1.pdf
4. More recently, this has has happened
https://www.techpowerup.com/330002/...-over-connection-to-china-backed-cyberattacks
There are so many reasons not to buy their products, despite the hardware not being bad.
See above. It's not a hardware issue, but there are a lot of other issues and concerns. That said, D-Link was in a similar boat some years ago and ended up in similar trouble, except point 4.