Been thinking about this banner and honestly I might agree with Sturdy Impact being overhyped. Not in the sense that isn't a fantastic skill (because it definitely is, and I'm still pulling for it), but people are ignoring just the concept of beast units in general.
They're 3 Mov units that get like three built-in weapon effects with no obvious drawbacks. Reverting to human form just means they function like a still really good Gen 2 Infantry/Flier. Like, Nailah's weapon is still Owl Tome X Slaying weapon without the transformation. The transformation just gives her Wo Dao and Atk +2 which is just gravy at that point. And it's not like it's hard to stay transformed, especially on an all-beast team.
It seems like they're forgoing a general stat-hike for Gen 3 in favour of beast units with their 3 Mov and silly Prf weapons.
Obviously they will probably add stuff like Beast Killer weapons and a skill or Sacred Seal that nullifies transformation effects to even things out, but beast units are looking like a real case of "you need to play around them" which could get annoying in Arena because if you build a team to take them on, they could get screwed by a non-beast team next match.
The way I see it, there's two notable weaknesses to Beast units:
-Beasts basically function like Dragons in that 'Beast' is essentially another class on top of their standard movement type. That means they get access to their own unique buffs like Hone Beasts so they can synergize with themselves, but it also means stacking weaknesses with their movement type. We will get beast-slaying weapons in the future, and once that happens, units like Tibarn will have to contend with both beast killers and archers (and Merric, I guess). Having exploitable weaknesses does help keep them in check.
-Transformation is easy, but it's not effortless. Players need to make sure that their beast unit isn't touching any non-beast/dragon units at the start of a turn. Depending on the map and your team comp, it might not be possible for them to transform right away. If you do stack your team with beasts/dragons, then you have a fairly predictable composition that people can prepare for. That's especially true for AR, where you can just have a reserve team just for taking on specific team comps.
And this isn't an inherent weakness of Beasts, but worth mentioning for one of them:
-Looking at Tibarn's stats, he's gonna be super easy to snipe. His attack is phenomenal, so with his followup ability and 3 movement, he'll tear up units in range. But his speed is middling and his resistance is poor. Any magical unit can delete him without too much trouble, and bulky blue units can likely still wall him (Adrift Corrin can even hilariously shutdown all his abilities with null-followup).
I bring this up because it's easy to look at that three movement and all those extra effects and think these guys are going to be nightmares, but I think we'll be alright. Tibarn is just a strong physical player phase unit. Your approach to dealing with him likely won't be substantially different from how you'd deal with similar units.
Overall, I think Beast units are going to be really strong, but I think they'll mostly be just powerful additions to the meta rather than something that completely changes how you approach matchups. Just my first impressions, anyway.
EDIT: Oh, minor point, but worth noting that it seems Beast weapons cap out at 14 mt like ranged weapons rather than the standard 16 for melee weapons. That's why they get +2 atk when transformed; they're (on paper) statistically weaker than normal melee units when not transformed, so transforming brings them up to the standard for their own weapon type.