I ran into an issue today where I could not save a template due to several errors stating 'Named Entity Expected. Got none.' on all urls that had utm parameters appended (for example, website.com?src=welcome&type=email. I cannot save the email with &, it has to be & which I question whether or not the form would accurately capture the value of that parameter?
I have never worked with an email tool that requires encoding the ampersand when it's used as a url parameter string.