Meh based on his comments his opinion seems to be mostly be Mawtribes are good, everything else is just kind of horrificly. It plays perfectly fine if you play in a "normal" enviroment. But in the far more competitive tournament enviroment where you have all sorts of nonsense they simply can't keep up.
Seems to be more an issue with everything else being broken than with them being "bad".
Also, it's a general issue with the competitive scene bringing out the worst in a game as people try to stack all the odds in their favour and thus broken/overpowerd/hyper-efficient stuff will be far more prevelant there. Which is normal enough, and the main reason why competitive tournaments aren't for everyone. The competitive scene, in nearly anything, will always be dominated by a handfull of effective strategies. Which in a game with a large variance in factions (or units, or champions, or whatever) like AoS will always result in most of those being fairly hopeless in the competitive scene. Which isn't even necesarly due to faulty balancing. Certain tactics, strategies & abilities are just going to be inherently better for the game, and for the factions to be distinct some of those need to utilise sub-part strategies. For example an army like the mawtribes, that relies on a very low model count, will always struggle in a game where bodycount matters a lot to score objectives. "Might makes right" might help them keep up, but the inherent weakness will always be there (unless you of course make this ability ridiculously overpowered...)
Another thing that really accentuates this is his tier-list in the second video. 15 out of 27 factions are "Very strong" or better. If over half your factions are "very strong" or better either the criteria for your tiers make no sense whatsoever, or your balancing is waaaaaaaaaaaaaaaaaaay out of wack and you consistently put in stupidly strong stuff.
Also, he then shows rather a large amount of potential lists which he claims at the very least are fun to play. And since noone likes consistently losing I'd have to assume they're at least halfway decent. Which is a bit at odds with the claim that the book is terrible....