I only know what I've read online. I remember reading that UHD players were very complex and that was part of why Oppo got out of the business- but that could have been speculation, not fact.
As for the players being problematic, I read a lot of threads in places like AVS and Blu-ray.com and there's is tons of evidence that many folks experience movies freezing- and the players locking up, requiring a reboot (ie unplugging) to work again. This is all anecdotal evidence of course. Some players seem more prone to it than others, but even top of the line players like Panasonic's UB9000 have some issues reported.
My Sony UBP 800X M2 had been very reliable, but recently it froze on the new Matrix UHD. A disc cleaning solved the problem- which is funny because the disc played through fine the first time. But upon inspection, it was filthy. My only previous problem had been one disc freezing momentarily, then skipping ahead a minute or so and continuing to play- which I chalked up to a manufacturing defect after a cleaning didn't help. Many of the problems seem to be disc related. There seems to be some QC problems at the disc plants. There's also a thread about the cases themselves off-gassing and causing dirty discs. Some folks report cleaning every disc before they play it.
Some triple layer discs seem to have problems freezing at what appears to be the second layer change- again some players seem to have this problem more often. There's a lot of speculation as to what causes these problems, but the fact remains that a sizable amount of users have experienced some problems. It's been said that the data is packed so tightly on UHD discs that there's little margin for error. Some players reportedly have problems consistently, while others with the same model report little to no problems. Speculation abounds about possible QC at the factories or possibly just a slight misalignment of the laser making some players problematic.
So- as to the first statement, I can't verify that, but to the second, there is ample evidence that UHD players- and the discs themselves- have many functional problems- though again, some users report problem-free playback. I was one until recently. I refer you to the 4K section of the blu-ray.com forum if you'd like to see for yourself.