It might not be legal, but works quite well for me.

Having a dynamic IPv4 address with multiple domains on it, updating every record would take more time.

Admittedly, I could also have a copy of the IP per domain, but the CNAME was easier to setup.

That would require updating too much software in the field.

Meanwhile you can use ALIAS record and get the same result, without others noticing.

There is no such thing as an ALIAS record. That’s something your DNS provider made up and shows to you in the interface, but behind the scenes they are providing the same old AAAA and A records as always, along with some sort of auto-updating feature which they will run to detect changes in the address of the name in the “ALIAS record”. How often this will update is anyone’s guess as it’s up to the DNS provider.

See: https://en.wikipedia.org/wiki/CNAME_record#ANAME_record

Sorry to be pedantic, but there is no such thing as an ALIAS record. It's just the DNS server doing the indirection for you.