It's incredibly smooth and powerful to use that feature. Set up some folders and it will auto-sort things based on the e-mail address, including sub-folders (e.g <parent>.<subfolder>@<username>.fastmail.com). It will also fall back on placing e-mail in the <parent> folder if there is no <subfolder> present.
I’ve run into sites that consider + invalid within an email address. It would also be easy for a site that wants to sell off user info to have a regex strip the +sitename from emails prior to selling the list.
The worst I've dealt with is a site that allows + within an email address for signup but not unsubscribe. They've been hitting my spam folder for years now...
Just in case you haven't tried it, I have found that about 80% of the time, they have some frontend JS regex that blocks the form, but using the wizardry of one's choosing, if you managed to get the email address posted to the endpoint, all is well.