Any trouble with the warranty support after flashing?
The primary problem seems to be a combination of design flaws and bugs that mean trying to lock the bootloader can brick the device to the point of requiring the device to be mailed to Fairphone’s service company to be reset. Fairphone refuses to fix the problems or provide a workaround (doing so would allow a serious unfixed security flaw to be exploited), and refuses to honour their warranty in this case, even if it happens through following instructions from their own support. They charge €34.50 for repair.
Realistically, however, given the security flaws with the device, locking the bootloader is likely not that helpful to begin with (the device trusts keys that have their private keys publicly available, and these are used to sign the stock OS).
Also, honestly, I see the usual “contact support” advice given in every context in this forum, and have seen few cases where contacting support has ended up being useful, especially for questions like these.
Can anyone confirm that the video output works on the non-stock roms?
Video output works fine with LineageOS. In fact, many features seem to work better in most non-stock ROMs than in stock, with a few exceptions for features that are problematic in the first place (eg, 48MP images).