Finished Riff 2 on Sunday! It was a great hack, I'm glad I listened to the little part of me that wanted to give it another shot. The levels are extremely well made, with almost no spots that made me say "this is bullshit". I feel like it's extremely rare for an expert-level hack to lack these, so that should give you an idea of the degree of craft that went into the level design. And I think it's only because the levels are so thoughtfully designed that the hack can get away with vanilla retry. (Yes, it's time to talk about vanilla retry.)
Here's the conclusion I've arrived at: for kaizo hacks where the player is expected to die repeatedly, sometimes within seconds of starting a level, I believe that vanilla retry is fundamentally disrespectful of the player's time. That's the reason I don't really ever want to play any real old school hacks that are intentionally brutal: I want to play the game for more than five seconds at a time. It's no fun to end up spending more time not playing the game than playing the game. But playing through A Plumber for All Seasons made me realize that as long as I felt like I was getting to play the level in between deaths, I didn't mind vanilla retry at all. One of the stated design goals for Riff 2 was to be intentionally lenient at the beginnings of sections for that very reason, and while it mostly succeeds in that, occasionally it doesn't. Those spots are where the vanilla retry becomes frustrating, but the hack generally works because it's designed around that constraint. I typically stopped noticing the slow retry unless I was dying early in a boss fight or something. I still think it would've made the hack better if it had fast retry, but at the end of the day it's still a really remarkable hack. Even the levels I disliked (and there weren't many!) were full of creative stuff to do.
I didn't keep track of my times, but I typically played in sessions ranging around 60-90 minutes, and I cleared 2-4 exits per sitting (usually 2 or 3). As a rough estimate, I'd say the total play time was around 20-22 hours? Difficulty-wise it's honestly probably on the lower end of expert hacks, and if you're not scared of vanilla retry it feels pretty approachable. Section lengths are extremely fair and it's really just a platforming hack at heart. I even ended up learning a couple of speed strats here and there when they made things easier. I've got nothing bad to say about it, really. If you're at all interested, and you've got enough Mario chops to beat something like GPW2, it's absolutely worth a play.