The difference is sv is melee... Btw I saw logs of a surv hunter in my server's first guild so it's not they're "unusable" in mythic. It's just there are so many other easier to play melee classes in the game. And most guilds have ranged shortage so they want ranged hunters.
The problem you noticed is low gear -> you're forced to do low content because no one takes you to high content -> in low content everything dies too quick because other people have more gear than you and kill it faster -> you can't do any dps and "get carried". You can only compare dps if you have similar gear to other dps and do appropriately challenging content so stuff doesn't die too quickly. I'm pretty sure +5 with people 920 ilvl and more will suffer from this issue, stuff dies too fast so classes with ramp up with show as "bad" on dps.
Anyway last time I've seen surv hunter in a dungeon he did really good damage by using butchery on aoe packs, kinda reminds me of feral druid and brutal slash, seems like very similar playstyles.
But yeah to compare anything you'd need to make a group of full 920ilvl ppl and then find a level of key where you don't kill stuff too fast yet it doesn't go too slow either. I'm pretty sure +15 would go really slow with a group of all 920s unless they're super good players, but +5 is too easy.
- - - Updated - - -
Funny cuz my guild had our last hunter quit during xmas and since then we can't replace him because there are just no competent hunters around. If there are any decent hunters looking for a guild, they always aim for a higher one. Last hunter we had on trial was absolutely disappointing, so even though we wanted one, he just wouldn't cut it. We have mages and locks and boomkins and ele shamans but hunters, just can't find any unless we lower the bar to the floor and that's not fair to other raiders. Glad tier sets are going away in BFA because seeing another warrior/hunter/shaman/monk token drop no one needs while the other 2 would be extremely welcome is just mockery. Out of 25 people we had 5 on that crappy token.