There's a few different things there, and I wasn't around for all of them. There's a bunch of threads on it if you take a look around, but they generally end up with a lot of bickering and interforum bullshit.
-misrepresented themselves as being a Chinese vendor (they say they make their own product, which I'm inclined to believe they do have a hand in the production, but others think they are a domestic reseller)
-shipped out reta that rapidly degraded. They replaced it for everyone that ordered, without requiring people to reach out to them individually, and people are currently waiting on the retest to see if the new product is better.
-some confusion on the filler used at one point, I don't really remember what came of that other than one product had something unexpected, and nexa panicked and assumed other products had it too. Turns out they didn't, but they'd already said the products did, which sowed further distrust among people who were already wary of them- why didn't they know what was used in the making of their own product etc.
-some people don't like the prices charged for the product, assuming it's actually from China, when you look at other group buys. But they're still significantly cheaper than any other domestic.
I'm sure there's other stuff but I don't want to stir up shit again and get called a shill because I like them. I figure everyone in the market is at least a little shady, and with Nexa I get it fast, I can use a credit card, I can personally ask questions of the owner, and it's being tested over and over, so if something goes wrong someone's going to shout it from the rooftops. Also, he wasn't hit by the latest zero pep thing, so he's probably at least getting from a different vendor than everyone else even if he is a reseller. All of the various Chinese companies are resellers too which was pretty much proven with the zero peps.