Earlier quoted context omitted.
No, the most annoying thing is when they don't tell you the rules in advance, and you start typing a password that fits most standard rules. Now either you get through but don't know if you could've used a better password. Or your password will be denied and you have to make adaptions or change completely.
No, the most annoying is when you paste your password manager's auto generated long password and it silently get cut after 16 characters but successfully let you create your account. But now the login form doesn't have the same max-length property and your password will fail. Hopefully the reset password form is synced with the login form.
I emailed support to ask and they said 'oh yes, we limit the number of characters in your password'. There was no documentation or feedback on the page to tell you this if you pasted the password, but if after pasting you deleted the last character and then tried to re-type it, you would see the error. Pasting never triggered the tooltip telling you the issue.
Now I try this each time I sign up for something which is a minor hassle but still much easier than the seemingly random failures I had been seeing. It also taught me to immediately sign out and test the services' password recovery functionality to weed out the ones storing plaintext passwords, so that's good.