@Ruin Explorer We have a couple of people telling that in actual play they had a good experience with the Mastermind Rogue. Have you played it or seen it played, or is this based on reading? It could be one of those that is unpopular because it reads like it doesn't play well, but that beats expectations when played by someone who enjoys the style of play. I've seen people say the same thing about bards.
I've played it (briefly), but that's immaterial.
When you're asking questions like this, honestly you need to think about the specifics of the criticism, and not be slip in the easy temptation to generalize and assume. You're doing exactly that - as shown with your statement re: Bards. It's a very extreme generalization. Also, personally, I've never heard anyone say
anything like that about Bards. They read like they're borderline OP, and as someone who plays them regularly, they play borderline OP (like most full casters).
My point is, look the actual criticism I've made. I'm talking about
specific issues within the rules that the class faces. I'm not saying "Yo this subclass sux imo lolz" or something where generalized "well people say that about other stuff!" would apply. I've specifically criticising
particular aspects of the subclass. It's unpopular for three reasons:
1) It's not a common conceit for a Rogue-type character, the Mastermind. It's not unheard-of, but it's not very Rogue-specific (for a D&D Rogue anyway - in PF2E it's a whole different class I note). So that already limits the popularity.
2) The mechanics don't really reflect the "Mastermind"-type character you might see with say, Kaz Brekker from the Six of Crows (or the recent Shadow and Bone Netflix series, which sorts of drops him in a lot earlier than the books), or Locke Lamora from the series of the same name. They make a vague attempt to - you can see the influence. But it's not done very well because it's not very imaginative or effective in terms of evoking a criminal mastermind. Almost any Rogue subclass could do that as well, arguably an Assassin (oddly enough) would be more effective in some ways due to Infiltration Expertise (and later Impostor), and an Inquisitive is more similar to some mastermind-types (albeit also to their nemeses). Part of this is because D&D 5E nearly totally eschews "narrative" stuff, which masterminds kind of rely on (interestingly, there is an adventure for 4E, which I often mention, Blood Money by Logan Bonner, which does allow these sort of narrative shenanigans, so the mechanical potential is there).
3) As I explained, the mechanics just aren't very effective. They're very "on paper" and very DM-reliant. The reason people are doing well with it is the same as a lot of classes in D&D 5E - the basic chassis is solid, even if the subclass doesn't add much. A Rogue without a subclass entirely would still be a
pretty solid class in 5E. The issue is of course than a handful of subclasses add a ton of mechanical power, and others, like this one, do not.
Re: Help, one thing I've seen on these boards is maybe most people misuse Help, at least by RAW/RAI standards. They let it apply to things it can't, they let people redirect it after the conditions fail to be met, they let it apply to multiple attack rolls, and so on. I don't actually criticise any of that. I think that's probably for the best, in fact! But that's another aspect of the DM-dependence this subclass in particular has.
Also it's not just about "enjoying the style of play". It requires the DM to
also enjoy that, and to providing an adventure environment where that works. Is that true of some other classes/subclasses? Yes. A small minority, but yes.