I think provided you have symmetrical lift from a multi bladed prop the flatbar shouldn't have much torsional load from the motor. In particular my suggestion was for the Mini Talon quadplane or similar build. Larger flat bars are probably going to be much heavier than tubes on larger frames, and probably won't be stiff enough and flex more.
The second build actually used aluminium flat bar as the quad arms, which was a bit flimsy and bendy, but it worked. I then went through a few weeks trying to get tubes to work properly, with nice mounts etc, even with the ESC integrated underneath the motor etc. but it was so tedious to build and repair, so I went back to the flatbar, and used CF instead, which is significantly stiffer than the aluminium was. I haven't looked back since.
The intermittent motor issue because of the ESC PWM was in fact way worse to solve. That took a solid working week to figure out, which resulted in me making a thrust test stand, and going through a series of ESC/Motors/props testing every conceivable ESC/PXH setting to find out what worked, and what didn't. It being intermittent and not happening under load made it a very annoying, blowy and very loud week of testing! (this was two weeks before the OBC!)