I've played a couple of AoS 3 games and as many other pointed out there's both pros and cons.
Note: This became quite a long post but I hope it's somewhat coherent
I like that the monsters have become better, because we have them and they're really nice to see on the battlefield. It's also nice that we have quite a variety - EoTG, Stegatrons, Carnos and Bastiladons, each with a different role.
Best thing so far: I LOVE that the EoTG is a priest.
I also enjoy the battle tactics, that's fun and it dictates your play in a quite nice way. I had a really nice experience where I managed to sneak away the skink unit that was nominated by my opponent to "kill a battleline". It feels that it's a lot more tactics going into this and the points gain/loss by them is usually significant and requires some planning to be able to accomplish your tactic each round and not be stuck with battle tactics you can't achieve in the end of the game (however, I need to point out that I'm not playing 40k so I don't know if this is just a step towards 40k or not).
The grand strategies have so far not made anything interesting. I have realized that this is quite an interesting target to be able to deny the opponent those sweet points by going after the grand strategy. But they also seem a bit to easy to accomplish.
I don't like the infinite armor save buffing. Yeah, it's capped at +1, but really not. The easy access to several sources of +1 just nullifies -1 rend so easily. For seraphon which is an army without much rend it's really tough and drives the play towards Mw's. This is annoying and is further driving the play towards Salamanders, Kroak and using the Serpent staff. I mean, when the Carnosaur has -1 rend, that's nothing in this edition and you can always count on your opponent to have an effective +1 when you finally get to bite them.
Seraphon might have an ok way of countering this. By having access to both shooting and melee it's possible to draw out CP's in both shooting/combat phases for the all out defense. But then you have mystic shield as well...
Regarding buffs I think seraphon is doing ok. Many of the "old" CA's and other buffs are very nice and since they usually stay activated until your next hero phase they're very useful. I've really enjoyed the skink priest's run/shoot/charge ability since it gives a stegatron a huge threat range.
I've only played Coalesced for AoS 3, so I don't have any realy inputs on Kroak/Slann, but since magic = Mws, they should probably be very useful.
One other note, I managed to play with the new rules from White Dwarf, and they're a very nice addition. The mystical terrain buff is handy and that primeval domain "overrides" mysterious terrain is good - even if you don't play with the mysterious terrain rules it's active. The coalesced mount trait for a Carnosaur is great, alpha strike on the smaller table size with run/charge is nice.
The amulet of destiny is of course nice, but as pointed out earlier it's too good. I've even found myself going for pure coalesced instead of thunder/koatl's just to be able to ignore the mandatory artefacts and taking the amulet. It just increases the carnosaur survivability by a lot.
An other thing worth noticing is the crazy amount of healing you can get. EoTG, lifeswarm, priests and heroic recovery. The survivability of our monsters is a lot higher. Especially in thunder lizard with that sweet +2 wounds on monsters.
What I think seraphon lacks, is rend. Mws are quite easy to get but it would be nice if your saurus with spears can actually kill something that's not just chaff.
Coherency rules is just what? Annoyance. It is theoretically possible to maximize the amount of saurus within 1", but it's just a hassle. It is possible to conga line skinks, but is that intended? Just annoying and adding time.
Finally, AoS 3 is bloated. So much to keep track on. Battle tactics, command points, regroup, unleash hell, priest stuff... And with almost every warscroll in seraphon having access to some sort of ability/CA it's just crazy much.
I think the monstrous/hero actions are ok, but it also leads to a lot of looking up. Is it a 2+ or 3+ on this thing or what? Is it + to hit or + to wound? On mount and/or hero?
I've not won yet, but I blame that on taking fluffy lists and inexperience against the factions I've encountered. I recently painted up my Carnosaur so I just want to put it on the table everytime (
https://www.lustria-online.com/threads/carnosaurs.13188/page-8#post-387850).
I'm going to play the Kruleboyz at 750 pts soon, and I'm very scared regarding their Mw output, so the amulet is unfortunately going on the carnosaur, again.
All in all I try to see the positive aspects. I usually play friendly matches and just try to roll with the punches. Seraphon is doing ok, they're not top of the meta but we have some good tools to work with.