Pretty sure it comes out IDed if you use a vendor recipe on ID maps, which makes it annoying as shit, I don't count that one.
I guess you're right, it's a few challenges off compared to last time where I did some stuff without hitting 90. Vaal bosses is the same as last time more or less though, only that it's not specific bosses and instead more random, but it's gonna be annoying to track I think. Mostly you throw a bunch of vaal frags in the map box, and then you track down the ones you need by doing triple quicksilver speedruns through the zone. It went pretty quick last time when I did it.
Unique strongbox I agree are super rare, but they're adding some, the warband ones. Or at least I think it's this patch? I know they're adding something like that. It's possible those are a decent bit more common.
Lvl 90/lvl 20 gems I've never done either, and yeah I didn't want to do it last league and probably don't want to this league either, but it's been getting easier as the powercreep continues(although it might be harder since they're nerfing xp gains now), but mostly I was trying to get it done in a couple of days rather than a full league. If I played like say, a week, I think I'd get 90 easily.
Twinned maps there were a bunch of people sharing them on #820 for free, and some selling them, so I would expect it'll be the same and not too hard to do. You can kinda blue roll a map of each tier until you get it, it's an uncommon mod but it's not like rolling a top tier damage mod on a weapon for example, takes like 40-50alts to get twinned at most I think. The main issue with the challenge is actually killing some of the twinned bosses, like when it's hard bosses. Although if you play an OP build that's not much of a concern. 8mods is a ton more annoying imo cause you need to vaal a bunch of maps and hope you get a what is it 1/8 chance? , and then that you can run the map.
So yeah I guess it's a bit worse, but not too much. Those endgame grinds are pretty fucking bad though, so don't know. Last league it was shit like IDing top tier uniques that was basically a "can you get 20exalts worth of currency", now it's more deterministic but also feels like more work. It's a bit like the vorici of leagues.