I guess my friends and I are a bit "out of the norm", b/c this was our gateway game into AoS (a few of us played Warhammer many years ago, but just fell out of it). All things considered, we rather enjoy Shadespire. Unfortunately, I actually haven't played Nightvault yet, so I can't really comment on the new rules, but my friends who have played Nightvault like it. There are positives and negatives about Shadespire/Nightvault.
To me, the positives are that the minis all look really great so far (especially the original Shadespire sets). The rules are pretty easy to follow and the cards give you options to "bend/break" the rules occasionally; you could have your entire warband wiped out, and still potentially win, depending on your objective cards. The games are pretty short; my friends and I can play a standard 1v1 in about 30 minutes.
The biggest negative is arguably that, similar to MtG, it's kind of a "pay to win" game; meaning that the more cards options you have, the better your chances. The only way you can get all of the cards is to buy all of the miniature sets, or pirate the cards online somehow. Second is that some of the armies tend to favor certain play-styles, and changing that up can make it a challenge to win. To that point, once you've seen a warband play, it will probably be the same style each game, unless you change your force or your opponent changes theirs.
Multi-player games can stretch the mechanics, but in my opinion, it's no worse than trying to play 3-person AoS or Skirmish. Again, to that point, I agree that the low movement teams can either get left on their board, or ganged up on if in the middle. But, in multi-player games many of the objective cards are changed in order to make it easier (or harder) to achieve them. For example, if an objective card says to hold objective 1, it will say you can score this by claiming objective 1 or 6 (in a 3-player game), or 1, 2, or 6 (in a 4-player game), making it a little easier.