One serious downside of mirror accounts is that the installed games in the library aren’t filtered to the current account (at least they weren’t last I checked). So if the accounts have different levels of purchasing activity, it’ll be a sea of not-your-games with some of yours sprinkled in between.

You can somewhat mitigate that with different sorting options, but afaik there is no option at all to hide games unavailable or unowned on the current account.

Functionally it works very well though.