The one longform comic I tried to read on tumblr had really bad navigation. Like, the next/previous buttons worked if I were just reading from the first page or the latest page, but the archive page was a mess, and even the next/previous buttons didn't work right if I went to a random page from the archive. Maybe it's just that one comic that didn't get it right, but yeah, that's my experience with tumblr + longform.
I agree with you on ComicFury. Their interface is kind of odd XD; but I really like how their forum is organized; they made it very easy to get comments, both quickish comments (that one thread in the self promo section) and the longer, more thoughtful ones (in the critique section). I would love to see both kinds of comment/critiques become more common over here at Tapastic.
In addition, ComicFury has a one-image-per-post limitation for your comic, which is a big problem for people with mobile friendly comics like myself. It's not an issue for page/book-format comics, though.
Another big difference between CF and Tapastic for me would be the comment notification. Keeping track of the comments on YOUR comic is fairly easy; not as effortless as it is on Tapastic, but it's perfectly manageable. Now, keeping track of the replies to comments you leave on other people's comics? There's no one single place that keeps track of that. You have to manually check each page you've left a comment on. That discourages meaningful discussions in the comments section, unfortunately.
While I'm still discussing ComicFury, I think another important difference between ComicFury and Tapastic -- or any two different sites of this nature, really -- is the vibe. Tapastic often feels like a bunch of friends hanging out and chilling, which is awesome. There seems to be a good balance of creators-to-'just' readers ratio here. But ComicFury feels like a bunch of creators mingling in one place, with not many pure readers. I don't have the stats or anything; it's just a feeling I get. Kind of hard to describe.
Hiveworks is one place that I don't get. They're not really a host, just a network (although they MAY provide you hosting if you don't already have one?). If they accept you, what are the perks? Other than being one of the cool kids, I don't even know! But I do love some of the comics on them.