<disclaimer: I am with Mender>

We have been evaluating OSTree as a potential building block for Mender, however the key challenges we've come across:

integrating OSTree into an existing device/OS seems quite invasive - https://ostree.readthedocs.io/en/latest/manual/adapting-exis...

block level signatures is not possible, which we feel is a requirement for an over-the-air updater

rollback is not built-in and can be quite challenging to implement reliably (bootloader level)

Think OSTree as more of a building block, like Git is for your development process. We might use it in the future but robustness and easy integration are our first priorities.