Personally, I don't get the hate One Small Favour gets, either. I think it's funny how you end up going halfway across Gielinor because one NPC you go to for help also needs a favour, so you go to another NPC who also needs a favour, and so forth. Personally, I'd love to see a sequel that takes you everywhere from Piscatoris to Mos Le Harmless and everywhere in between.
As for Mournings Ends 2, most of it is devoted to one huge puzzle which, even with a guide, is long and time consuming. And unlike later quests' similar puzzles (such as Song of the Elves' Grand Library puzzle, where checkpoints include some Seren lore drops), there aren't any lore tidbits placed here and there to liven things up. So yes, the puzzle can get old pretty fast, especially if you don't have a handy guide with a checklist to 'check off' all the steps.
So while I don't get the hate One Small Favour gets, I DO understand the hate MEP2 gets. It's certainly one big puzzle that requires a LOT of patience.