What downside, if any, is there to using Unbounce for your homepage?

what features does it lack for SEO ?

As of my experience when using unbounce for all our organic traffic was not very effective the main problem I encountered was the lack of blocking certain inbound links from spam sites which affected rank score, their IPs were different all the time so it was a nightmare.

Also hosting some pages on a subdomain was considered as a totally new domain so all the SEO efforts I made on my domain didnt affect the unbounce page so I tried to copy the content but then it ranked lower because there was duplicated content.(not sure how unbounce deals with duplicated content with variants though)  It does covers all the basic metadata descriptions but not enough for constant SEO optimization. 

But I think if you have reviews from a Google review partner pointing to the unbounce page then you will rank really high. For us, reviews have made us rank on the first page for some search terms. 

As of SEM, Unbounce is great because the content is focused and Google Adwords doesnt care much about inbound links or duplicated content on other sites so quality score for my paid campaigns is higher which leads to lower CPC and lower CPA so its good for direct marketing, also you dont need to deal with the developers to make changes so everything is really fast with Unbounce.