• The process of selection has a usability problem in case there are more then 20 users: "Select all" button doesn't select all, but all users listed in current page.
This might lead a teacher to mistakenly think he send a message for all users when he actually sends to 20 users that appeared at the head of the list.
• Anyway, after selecting the users (and before sending a message to them) there is no appropriate message that says how many users were selected.

https://moodle.org/mod/forum/discuss.php?d=312943