[jeffbee]

Everything that is normally annoying with IPv6 literals, plus the fact that it's encapsulated as [IPV6:<the address>] and the address could take a dumb form such as ::1.2.3.4. But I mentioned it because it might initially seem to a neophyte in this field that the thing to the right of the rightmost @ sign is a name you can pass to your resolver. It might not be.

[kazinator]

The IPv6 notation claims : for separating digits. But that's already an established notation for port numbers: 10.1.2.3:456. Oops!

The square brackets allow us to stick a port number on it: [ffff::0123:4567]:6301

[jeffbee]

Yes, but the RFCs permit 6v4 addresses such as ff:ee::aa:1.2.3.4

[jcranmer]

Note that IPv4 address literals are encoded in square brackets (e.g., user@[127.0.0.1]), so it's really all IP address literals that could be a problem rather than specifically IPv6 literals.