Agreed. I don't think they're particularly good or bad - just meh - but when it's just bonus content it doesn't really matter. Like you say, if that was the entire game, it'd be a huge issue (part of the reason I'm currently not enjoying Nioh very much), but it isn't so I don't know why people criticise it so much.
Side missions aren't essential of course, but I know if I don't do at least some of the side quests I'm going to fall behind in terms of levels (and despite what everyone in the 'get good' camp says about levels not being important in these games, for anyone who's not a speedrunner or wannabe pro they absolutely are).
Cut out the side missions and balance the game around original maps only and Nioh would be so, so much better.
Not sure if it was my advice to skip Sekiro. I did say to go into it with a different mentality, though!
as for Nioh, I thought it was okay. 7/10 for me. Good, well made, but it's got elements I don't like (randomized loot, for example) and something about it felt sterile and stiff even if it totally wasn't. It lacked the depth of lore and world building that Fromsoft's games have. I played it, I made it about 25% through, and then moved on. Very long game that I just didn't have the heart to complete. I seem to be in the minority, though.
My Console Library:
PS5, Switch, XSX
PS4, PS3, PS2, PS1, WiiU, Wii, GCN, N64 SNES, XBO, 360
3DS, DS, GBA, Vita, PSP, Android