It's probably mostly true. But some of it depends on your play style and the type of build you have. Aldrich can wreck certain builds, or certain play styles, especially if you're new to him. Pontiff Sulyvan is one I'm still not confident of while going in, and I honestly can't remember if I've solo'd him yet or not (there's 3 NPC summons outside and they're just too tempting). And The Dancer works a lot like Aldrich, she can easily catch certain builds or styles off guard, and there's plenty of videos out there that showcase how off rhythm her style is, so its pretty common that she throws most people off their game once or twice. But mostly yeah, Nameless King is the beast to beat. Though unlike O&S he's optional, and I'm not sure O&S were designed to be "the fight". Nameless King was designed to be "the fight". So it probably doesn't resonate quite the same. There's no easy parallel in the game that I can think of. Plus there are DLC fights that are contenders