The deal with ZTE is that you NEVER buy one of their products in the first 6 months to a year after they are released. They do the absolute minimum platform validation and count on customers buying them at their low price points and not caring about the problems. Once they discover from their customers what's failing and costing them money to replace, they do a mid-cycle revision and fix their biggest problems, which is usually out to customers in about a half a year after release at the earliest. The other thing to have in mind with ZTE is that, whatever OS release is on their phone at RTM, that's what it's going to have for life, save for a few minor security and stability updates here and there. You buy a ZTE phone with the idea that it's going to last you about a year or so, that it's decent specs for the money you're paying, and that you're going to replace it with something better a year or so later.
A case in point: The ZTE Blade ZMAX Pro. At release, it had a VERY high return rate. T-Mobile and MetroPCS were having to replace them by the truck load. I have two friends that work for cell phone stores and both told me that after about 6 months, returns on those phones dropped dramatically. I went ahead and bought three of them for my kids, outfitted them with chunky cases, and they've all been doing just fine with them for most of a year now. None of them are showing the problems that early phones had, no strange crashes, no bad screens, no strange behavior. Just solid service. The only one to give any problems had a stuck volume button from having it out of the case, which was easily fixable. Those are solid mid-range phones for under $100 each after the carrier subsidy, and under $170 without.
Just know what you're getting when you buy them, and don't expect it to be a flagship device.