Let's take a look at one of the achievements added in 8.3:
https://www.wowhead.com/achievement=...ption#comments
In essence, it's a "kill x of mob y to spawn mob z". As grindy as it gets but that is often the case. Except this achievement way way way overdoes the grinding. I spent 9 hours killing jailers and an additional 12 hours killing watchers until their special versions spawned. Not to mention God knows how many hours were spent on the assortment of other mobs.
That's easily 40+ hours of work for 10 achievement points.
Let's get some things straight. This achievement is entirely optional and will only ever be done by collectors. But collectors are the people who would like to collecting EVERY achievement, EVERY pet and EVERY mount. By creating insanely grindy, hard-to-complete achievements like these, Blizzard is pushing their target audience away. There comes a time when being a collector isn't worth it anymore because you can spend those 40 hours working on other hobbies or playing other games. Why spend so much time monotonously killing mobs in an RNG-infested hellhole when you could be having fun in the Last of Us 2?
Hard, grindy achievements should be rewarding (more points, mounts, titles or pets). If they aren't rewarding, they should be easy. Killing 1000 mobs on a 6 minute respawn timer for a 0.1% chance that one of them will "corruptforge" is not fun or rewarding. Just like most other BfA grinds.
SL has to ease back and make collecting everything easier or it will just burn out more collectors.