Plasmic shows more promise as a solution for designers on product teams to take over CSS styling on the product itself (eg. your SaaS app with a react frontend). But I think they still have a long way to go on the visual editor...last time I checked their editor didn't even work on Safari.
Using Plasmic + JAMstack on a marketing site would be the worst of both worlds IMO. All the static site hassles with none of the stuff that makes static sites fun for me.
It gives design and marketing the ability to edit landing pages, but they still have to bug the engineers for bigger changes to the site structure.
The biggest advantage to Webflow is its super versatile CMS, and separate simplified editor for content teams. Our engineering team literally never has to touch anything on our main domain. Marketing/Design completely owns it. Our engineers focus entirely on our product.
Like all developer-centric tools coming out of the JAMstack ecosystem, Plasmic seems to ignore the existence of content marketing and SEO (Wordpress's writing experience and friendliness for non-devs is why they won the internet), which is the entire point of having a marketing site in the first place.