I guess that I haven't read the source code to make sure there's nothing malicious there? I'm kind of a scrub, which is why I decided to give this thing a go in the first place. I say "seems to take care of reverse proxies and stuff" because I haven't checked at all to make sure any of that's working. I've done no pentesting either. It's not that I can't figure out how to manually configure proxmox or whatever, I'm just usually too tired to put in the concerted effort, so Cosmos has allowed me get things up and running quickly and without having to learn too much more than I already know beforehand.
Also, Cosmos does take care of basically everything by itself, but when I first set it up (many patches ago now) there was some issue with the way it assigned UIDs in containers so that the root user in some containerised apps couldn't see the data even though it was in directories that were correctly bound to the container. I had to enlist a friend with more experience to help me troubleshoot that. So, defaults are usually fine but it's happy to let you shoot yourself in the foot if you don't really know what you're doing.