I've been using it on the XF forums I manage for some time now. No issues at all except for the brief MariaDB debacle, now fixed, which had nothing to do with PHP.
Just make sure before you upgrade PHP to check the main page of the Xenforo ACP for a list of modules you need to enable with PHP 7.4.x. That's not automatic: Just because you had them enabled in in 7.2 doesn't mean they will be automatically enabled in 7.4.