The very one.
He’s a massive pain to solo. Doable, but never easily so. Generally, not worth messing with unless you’re really down for some achievements, pets, or transmog gear. Or feeling very touristy and totally want to take in the ambience of BWL. 
TBH, BWL is just unpleasant to solo even as a 90 on many levels.
We’re discussing Razorgore. We all know he’s a PITA.
Then there’s Vaelastraz, the next boss, who is guaranteed death to any mana user unless you can burn through his 1.92 million HP in 35 seconds or less. (My calculations make that 132k DPS. I can do that on my well-geared hunter, but it’s absolutely beyond any of my alts. Ironically, since hunters don’t use mana any more, it’s not actually a problem. But it did kill my enhancement shaman when I tried him.)
The next boss, Broodlord Lashlayer, is just annoying. Well, not him, but the room leading up to him, because it projects an apparently unavoidable slowing field that makes you move through it at no faster than walking speed. I’m not a patient man, and RP-walking through a room full of annoying but harmless whelps just pisses me off.
The drake bosses after Lashlayer aren’t bad, but Chromaggus can be annoying if you don’t honor the debuff mechanic he has: if he can lay all 5 debuffs on you (take too long killing him, and don’t clear the one that you can with the item provided in earlier kills), he’ll MC you and reset. End of encounter, enjoy trying that again with all the debuffs you’ve already accumulated. :mad:
Nefarian, the final boss, never really bothered me, but there are some mechanics that can be annoying. The “class call”, in which every 30 secs Nef does something bad and specific to a class, is particularly annoying to Hunters: he instantly and completely breaks your equipped ranged weapon. Which means either the hunter must swap out to a spare, use a repair bot, or punch him. (BMs are better off, since the pet can still do a lot of damage.) Some of Nef’s summoned mobs have an MC mechanic, but I’ve never seen it… either it’s disabled for soloing, or else I’ve been doing well enough burning down summons.
BWL is probably the hardest soloing experience in the classic game. I don’t blame anyone for seeking out a second.