A few things:
1) Behringer had a reputation for poor quality and unreliability dating back to at least I think the 90s. In my experience that’s no longer fair criticism, at least for the products I’ve owned, but the reputation persists.
2) Ethical issues. They mass produce using cheap labour in the far east (I forget exactly where). There was some controversy involving a synth/instrument sponsorship a few years ago that took an excessively long time to materialise. They are perceived to rip off other companies’ IP and undercut on price because they don’t incur the R&D costs. I seem to remember some noise/drama around one or two partnerships as well - maybe the UBxa? Again, I can’t remember.
3) They are a budget brand whereas the synth and pro audio communities are somewhat overpopulated with vocal gatekeepy snobs. I’m sure they’re probably a minority but, at least in the online world, they’re often loud and visible. You will, for example, be pretty reliably downvoted for saying anything positive about Behringer in the Reddit synth communities.
4) Uli is perceived to be a bit of an odd duck, again, by people who tend to make noise online.
For myself, I don’t love everything they’ve done business wise, but they make a good product that won’t break the bank for those on a budget. I have a couple of their TB-303 clones, which are very good (loads of companies make 303 clones, often known as acid boxes, so I don’t really see this as controversial). I also have a Poly D, which is based on the Minimoog, but has an extra oscillator, paraphony, MIDI support, and a sequencer, and I wanted a taste of that without having to drop £5k on an instrument with, by contemporary standards, very limited capabilities.
There are a bunch of synthtubers I enjoy who seem to have a good opinion of their stuff: Wine & Synths springs immediately to mind.