Wrong hand stack ETA 2836-2 is very reliable movement, if you really get a genuine ETA. I've owned many of them and never experienced any problems.
The GMT function is faux, read this, it's not a "real" GMT movement, but serves its purpose.
The faux GMT function has been achieved by using one extra gear, which is 2x bigger than the hour hand gear, so it slows down the hand 2x in relative to the hour hand. This modification is so simple that it shouldn't cause any kind of technical issues.
I've heard that sometimes the GMT hand doesn't follow properly, but this is often just complaints from people who don't understand the function of the GMT watch. It is supposed to be twice as slow as the hour hand, and on GMT Master you read the 24h time from the bezel.
The new wrong hand stack movements in the GMTIIc seems to ba hit and miss, and I have no idea what movements the dealers put into these watches. They might not be genuine ETAs from all dealers... but in the old days this movement used to be good.