The main thing you lose with them are UX for sending newsletters, handling bounces on those lists etc. that's what sendy covers.
I found it incredibly frustrating that there was functionality that should have been easy for SES to implement, and that pretty much everyone will need at some point, but they don't really make it easy to add on after the fact.
SES uses SNS for bounces, complaints etc. So _really_ you need a http(s) endpoint that receives these notifications and can act on them in a way that you'd like. (Unsubscribing from a mailing list, analytics, etc).
I've used phplist feeding to AWS SES for many years now without issues.
Except MC doesn't provide this one, mission-critical feature of inbox delivery.
Most of my friends and family that subscribe to the PhotoStructure newsletter found their email in their spam folder.
And yes, I set up all my domain records properly, at least according to MC, and there aren't any warnings or diagnostic failures on their side. Each campaign has said there was 100% delivery success.
There are quite a few messages that never even make it to the spam box, which I have never understood because plenty of very obviously sketchy spam messages get through to my spam box. Yet, occasionally we have trouble getting very legitimate transactional emails through to some customers.
You are paying for glitzy UI, marketing, and some nice features as far as email themeing and then of course, support.