Zotac was for a long time, 2nd tier gpu, along with PNY, Galax and others of similar values. 1st tier being Asus, Msi, Gigabyte, Evga and Sapphire.
Zotac, like most 2nd tier gpus did one thing. Took a reference board, jacked the clocks to the hills, and slapped on a different cooler. No custom pcbs. Just wild claims on performance and cooling. As a result, you got high performance cards, but at the cost of little OC room, and very touchy with temps and excessive speed fans.
It's only in the last couple of years that Zotac and Galax etc have stepped up and designed custom pcbs for the cards, which has the advantage of being able to use better spec'd parts for the higher power draws.
Zotac did have reports of funky QC, but that was...