In the last two years I have been mostly playing non-Legion content. And let me tell you, non-Legion content is pretty buggy. Not so much leveling quests but "end game" for each of the expansions. Whenever I approach one of the level breaks and do former end game content on them, I know that something will fail to work.
Every time they add something for the current end game that touches old content, they end up breaking some of that old content. Ie, when they added Call of the Scarab more than a year ago, they broke the level 60 PVP quests in Silithus. That has never been fixed. This seems a common story. It looks like they have been breaking a thing here and a thing there, and then leaving them unfixed over the years, and it accumulated to a pretty big account.
The buggiest piece of content by far that I encountered is WoD garrison. The last two months I spent there and I am encountering a new bug nearly every day. NPCs selling things, stationary ones that should always be there, not spawning (you want to buy something? good luck relogging five-six times trying to make the NPC spawn). Items not working (you try to reroll two undesired abilities on a follower - the first reroll replaces ability 2 with ability 1, then the next three rerolls do nothing, 1000+3000 garrison resources down the drain). Quests vanishing from your quest log when you portal to the garrison or even at random in the wild (you pick up a quest chain, you do quests 1-2-3-4-5, the final quest you have to hand in to the NPC in the garrison, so you portal to the garrison, and bam - you no longer have anything to hand in, the quest that you just completed has been removed from your log).
At one point I just started ticketing every issue I hit the moment I hit it. This obviously got nowhere because the super-cool Blizzard support is - obviously - not quite as super-cool as the vastly outdated urban myth paints it to be. In some tickets the support guy was busily trying to cancel or close the ticket by not reading it (ie, with the issue of vanishing quests above the response was this: "I can see from the log that you abandoned the quest! pick it up again and all will be well!", it came two days after the ticket was filed, and the ticket was canceled which made it just disappear from the in-game UI - and yeah, no kidding, the quest was perhaps abandoned but the entire issue is that I did nothing to abandon it, it's the game that abandoned it for me automatically due to a bug). The remaining tickets ended with "yeah, this looks like a bug, if you want to report it, you can, but no, we don't care that you cannot complete an achievement due to it and we totally don't care that you wasted an hour of your time / resources / whatever here, we won't compensate shit".
Maybe Legion content is better in this respect, but outside of Legion, Blizzard polish is just not there anymore. It's a buggy game and you are on your own.