Firstly we consider that there are multiple replication possibilities of both technologies- however I'm going to assume the defaults because that's pretty much what everyone uses except if there's an actual case for using something else. It's the exception.
But by default MySQL uses statement based replication (in a weird binary format with log positions and stuff) and postgresql does logical replication (as in, you transmit the binary differences of what you'll be doing to the replica's database files directly and the replica just follows along)
Both of these approaches have trade-offs depending on what you want.
Statement based replication is great if you want to have _different_ datasets on each side, You can transform the data or remove huge chunks of it on a slave and use it for a dedicated purpose. However that applies the other way, you can never really be 100% sure that your replica looks exactly like your master.
this bit me a few times with MySQL when I assumed that because the replica was 'up to date' with the master and it was set to read only, that the data had integrity- it absolutely did not.