For me the reason I use mailcow is it's a few commands and I have a fully functional mail, calendar, and contacts server with a web UI. It can then be updated with a single update.sh.
Using mailcow I don't need to mess around with virtual domains, connecting it to mysql, DKIM, etc which is far more than a few commands.
I also get a nice admin UI, web mail and CalDAV and CardDAV which you don't get with the above.
Also I'd consider the separate containers a good thing so that if rspamd, or clamav, for example, has a CVE they can't pivot to other parts of your mailserver.
Why? three main reasons: (a) security (as you have identified isolation matters, but that is not the only thing), (b) get the benefits of "battle-tested" setups and (c) features
On security: in its default config, Mailu scans emails for malicious macros via oletools (and optionally viruses via clamav). It also uses a hardened-malloc, Snuffleupagus (a security module for PHP), gates all PHP code behind an authentication wall (webmails), ... and does both DANE and MTA-STS validation to ensure your emails are delivered to the right place. The authentication stack handles "smart" rate-limiting: you get to limit the number of authentications with distinct credentials over a time-period (a misconfigured thick client won't trigger it), you have plenty of ways to avoid running into it (application tokens for thick clients, per-device cookies that give you a way out, whitelisting of "used" addresses, ...) and you also get to rate limit the number of sent emails (useful if a spammer gets their hands on the credentials of one of your users)
On the importance of "battle-testing" setups: well, there are plenty of non-subtle ways of breaking an email setup. Experience has shown that all the layers in the stack can be problematic... I can give you a bunch of examples of what we ran into recently if you want.
On features: your setup might be simpler but your users are missing out. Whether it's enhanced filtering (like with oletools), better indexing (full text search), indexing of attachments (with OCR! via Apache Tika), configuring server-side rules with managesieve or just "having an interface" to configure ooo, change their passwords, configure aliases or delegate permissions.
I have started spending time on Mailu because I don't like the bloat that comes with Mailcow. Give Mailu a shot; it is reasonably easy to debug when things go wrong (and not written in PHP :p).
In fact, I just added another domain to it.
The key here probably is, most people don't want to spend time learning enough about Postfix, Dovecot. Etc. To do that.
I've been running it for ~5 years this way and it hardly ever lets a spam through (Very rarely the spams that are just images get through) and I've rejected 2 mails that I'm fairly sure were legit (just bulk marketing ones though)
People always say "Use Mailcow!" and I say "But why?". Adding the additional complexity of docker with the random firewall rules it adds, individual containers that need to be fed and watered. It's a mailserver, all the processes are doing the same mail related function. Don't get me wrong, there's a very real place for Mailcow etc, but for a small end-user mailserver I think it adds overhead that makes running a mailserver more complex, not less.
Anyway, great in depth article.