With Windows 10 Mobile Microsoft seems to have taken an attitude of little handholding and letting users make mistakes if they want.
While we appreciate this less paternalistic attitude, it does mean the need for a lot of warnings.
The latest is that Windows 10 Mobile Technical Preview users should not try and uninstall the Outlook Calendar and Outlook Mail apps, no matter how much you hate the hamburger menus.
For one, it will not bring back the old mail and calendar apps, and secondly, you will end up being stuck without any mail or calendar apps, as there does not seem to be any way to re-install them without hard resetting your phone.
So, dear Technical Preview users, walk carefully around those landmines!
Thanks Critical Thought for the tip.