With WP-CLI 2.3.0, I accidentally typo'ed the --user_pass param to "wp user update" as --user-pass (i.e. a minus instead of an underscore). Here's what was output:

wp user update myuser --user-pass=mypass
Success: Updated user 1.

However, it didn't set "mypass" as the user password (it left it as the old one) and didn't report that --user-pass isn't a valid flag. When I used the correct --user_pass flag, I got the same success message but it did change the password to mypass. It needs to give an error message about --user-pass being a bad flag instead of a success message. I didn't check if this applies to similar typos with other user update flags (many of them have underscores).