You already got answer regarding crashes, my reply is about Feedback in TestFlight.
This seems to be old question but people keep asking it today.
I also had it and contacted Apple Developer support about it and they asked me the following:
1) Do You have Feedback e-mail configured in Testflight?
I had it configured of course so they only answered demanding screenshots, app ids, e-mails , e t.c. and other proof that I am not kidding. Then they asked me second question which I think sheds light:
2) Does Your tester have Apple Mail configured?
"... the tester should have Apple Mail configured on the device from which he can send an email. (It does not mean that the email address should be an Apple ID, but it should be configured with Apple Mail)."
They mean, Apple MAIL app that comes with iOS. That's how TestFlight feedback works - by fetching sender's e-mail from Apple Mail app when tester taps "feedback" button. No configured Mail app - no Feedback button.