This is the first modpack I've ever "finished" (I stopped after I broke out of the bedrock box).

It's been a while since I've actually "beaten" this modpack, so this is more of a retrospective rather than a hot-off-the-press review, but Compact Claustrophobia gave me a similar feeling of nostalgia that playing Minecraft beta as a kid gave me for some reason.
In terms of progression, it is one of the most innovative packs I have ever played and every step of the way to completion felt like an engaging puzzle to solve. It's grindy enough to make you stay a while, but not grindy enough to make you feel like you're doing chores if you play your cards right. Additionally, every time the modpack started to become a bit tedious, there would be a perfectly placed breakthrough tech (e.g. bigger compact machine or better way to make previous compact machines) that allowed you to do previous things in a much easier fashion which helped tremendously to keep me playing. I haven't finished the final completionist chapter yet, but from what I've heard the pack falls off hard after you break the bedrock box (as someone else said, literally a useless DLC) so I'll count my run as complete because that was my goal all along. The pack has a clear end goal and is a good challenge to sink over 100 hours of time into.