Skipping over the fact that most email providers don't necessarily support this, and that not all websites/services will allow it (not saying they are right not to), this creates a whole new set of things to remember as even something as simple as +hn (which is surely simple enough that anyone could guess it) could be tougher on other sites, e.g. is Reddit +rd, +re, or...?
To carry this idea further, the +descriptor email trick has backfired on me before. Sometimes sites require an email login and it takes me several tries to even remember what my +descriptor was.
Sometimes, I don't actually remember and just end up going to my inbox to find an email from the site so I can look up what I used.
Another time, this backfired on me when I purchased concert tickets using a +descriptor email and couldn't login. The purchase form and login form had different validation rules, so the purchase form accepted my +descriptor email and charged me for it. The login form rejected my +descriptor email and I was rushing to contact customer service to print my ticket in time.