For me, it's about pace. Sláine is one of those Mills creations that can work when it's being relatively ponderous and epic, but also in quick, distilled blazes of action (although we've not really had much of the latter for a number of years), and Savage has a lot going on for the most part, even if I can imagine us seeing 'end of book 27' a number of years down the line.
Flesh: perhaps it's just expectation. For me, Flesh should be all-out ballsy classic 2000 AD. It's about cowboys and dinosaurs, with mad time travel and a dash of satire. Early Flesh had an exciting energy about it. And although today those strips lack maturity, they're still a lot of fun. Flesh today is a trudge. It'd be interesting if you had an artist with the energy of McMahon or Flint, say, with similar scripts that were compressed into half or a third of the space. As it is, it remains for me one of the biggest disappointments in the Prog, and something very close to a skip.
(As for the art: I don't think that's the problem unless Langley's somehow demanding he have loads of massive panels and splash pages. It does feel a bit murky at times, and a little stiff, but there's some great stuff with dinosaurs and design in these recent episodes. The problem is the pacing. It all just feels so slow.)