It's actually not too difficult in MySQL, in terms of the mechanical steps required. MySQL's built-in replication has always been logical replication, and they've made major ease-of-use improvements in recent years, for example:
* efficient binary data clone is a single command
* starting replication with GTID positioning is a single command
* data dictionary / metadata upgrades happen automatically when you start a newer-version mysqld
The hard part of major-version upgrades in MySQL is testing your application and workload, to ensure no deprecations/removals affect you, and checking for queries with performance regressions. Tools like Percona's pt-upgrade and ProxySQL's mirroring feature help a lot with this.
haven't used mysql in a couple of years but its replication (all methods) used to have a whole page of gotchas and idiosyncrasies with various corner cases.
They also introduced binary file format changes even with minor and patchlevel version number changes and downgrading stopped being supported. afaik in that case had to restore from backup.
it's just the exact opposite of postgres' upgrade guarantees.
It's hard to respond without any specifics, but in my personal view it's a pretty solid replication implementation in modern versions. Sure, there are some gotchas, but they're not common.
Realistically, every relational database has corner cases in replication. There are a lot of implementation trade-offs in logical vs physical, async vs sync, single storage engine vs pluggable, etc. Replication is inherently complex. If the corner cases are well-documented, that's a good thing.
I do totally agree the lack of downgrade support in MySQL 8 is problematic.
Postgres is a really great database, don't get me wrong. But no software is perfect, ever. Consider the silent concurrent index corruption bug in pg 14.0-14.3 for example. If something like that ever happened in MySQL, I believe the comments about it here would be much more judgemental!
What about replicated and zero-down time (the current thread context)? That's also hard with MySQL.