Its a problem but it isn’t a major problem. I am using rspamd without any sort of exotic configuration (basically just enabling things that are provided, not my own rules) and I only get a few spam messages leaking through a week. Maybe slightly worse than GMail but not considerably slow.
IMHO the only real missing thing out of the box is contacts checking. Which is a huge thing because it is great to have reliable delivery from contacts. But my false-positive ratio is so low anyways that it isn’t a big issue and things like the known_senders
module mostly mitigates it.
Reverse DNS is different than static IP.
But yes for outbound email, if you can’t control reverse DNS you will have pain. (Inbound is totally fine) You can in theory just use whatever hostname the ISP’s reverse DNS resolves to however you will get some spam score (or be rejected) as it doesn’t match your “from” domain.
Outbound email is a huge pain really no matter what. Unless you have a long-term lease on the IP and it isn’t in a bad network you really have to pay someone else if you want reliable delivery.