If we're going to judge Seattle prospects based solely on a couple of Memorial Cup games, then I'd be much more worried about Guenther than Lambert.
I watched most of Seattle's playoff games, and Lambert was a strong player, even when he wasn't producing a bunch. He's obviously very strong in transition and with puck control in the offensive zone, but I was also impressed that he wasn't puck-dominant, and was a willing and reliable passer (in transition and in the offensive zone).
He's still just a D+1, so he has plenty of development runway ahead. I think that coaching will limit his high risk plays, playing at higher levels will enhance his give-and-go game, and getting bigger / stronger will make him better all-around.
For me, he's moved from a risky boom/bust prospect to a promising prospect with NHL potential. I would note that some of these positive traits were seen in Jets' training camp and in the AHL before his COVID bout and some injuries.