You should try it again instead of speading very outdated info about it. All major clients and server implementations have more or less feature parity and interoperate fine these days (and yes there are yearly complicance suites for XMPP that are exactly what you are asking for). What you are saying was true 10+ years ago when Matrix didn’t even exist yet, and Matrix has very similar issues with different client and server combinations these days.
You should try it again instead of speading very outdated info about it. All major clients and server implementations have more or less feature parity and interoperate fine these days (and yes there are yearly complicance suites for XMPP that are exactly what you are asking for). What you are saying was true 10+ years ago when Matrix didn’t even exist yet, and Matrix has very similar issues with different client and server combinations these days.