The Brain controller is a breakthrough compared to other flight controller as it is relying on the F4 chip so as to drive the OSD. By contrast most other boards are using a dedicated chip that is good enough for text based OSD. In order to render the graphical display, it looks like Martin just merged the dRonin/Taulab OSD into Betaflight, which is quite a bit of work.
At the end of the day the brainFPV OSD for Betaflight is quite unique, and no doubt it takes quite some time to keep the code up to date. As an added level of complexity, the F4 chip has to cope with both the Betaflight code and the OSD, so things can easily happen if the new Betaflight code suddenly becomes power hungry.
Let's be fair to BrainFPV: 1) this is a different product with a very specific architecture 2) it's got some unique features 3) Inav support is also offered 4) it's hard to make money in this niche market ...