From a security perspective forms in an email are something to definitely avoid - even if your testing had given different results and shown that they could actually be made to work across all of those email clients.
There would be no way to tell which spammer you were transmitting the form to (unlike links to web pages where at least some browsers will give you a some idea of where they really go if you hover over them).
If using forms in emails actually worked then all spam emails would include forms to collect information from anyone who is misled into opening it and filling out the form. Not only would the spammer confirm their email address (as they can if the person clicks a link in their spam or allows remore images) but they also get the information the person typed into their form.
Those email clients that refuse to display the form are far more secure than those that simply displaya dialog telling the person they can cancel submitting the form.