As much as I love Mailbox's flow for handling my email, I can't get over the performance issues and random crashes with the app. After a few weeks of usage the app slows down on launch and becomes a chore to use.
In contrast, Outlook (previously Acompli) is very stable. It may require more taps to do something like move an email to a specific label, but at least the app performs well and hasn't crashed on me once.