I have a flow triggered by an email arriving in a shared mailbox. It triggered just fine on a bunch of test messages on Friday and they all ran successfully.
Then come Sunday, 17 emails arrived. The Flow only picked up 14 of them
Update: I learned today that a colleague manually copied 3 emails into this shared mailbox from elsewhere. That's why they didn't trigger any flow. Phew. So, my concerns about reliability were completely unfounded.