I wouldn't necessarily call it unreliable, but it requires a lot of setup. Setup in software and hardware. Given what you describe, my guess is that it's still one of two areas for you to look into: the software setup AND/OR the power setup.
For power: There are two types of minimOSD's, the original has a digital (the telemetry input side) and analog (the video input/output) side. The digital side required 5V and the analog side required 12V. If you have a newer/modified version that gets rid of the 12V side power requirement (usually by skipping the built-in buck/linear converter) you can save on some heat and wiring complexity (assuming your FPV gear is also all 5V). Anyway, I think a lot of problems stem from this complexity. So, the question remains, which version is yours?
For software: oh where to start? First off, there are multiple software that will work on minimOSD. Probably the two most popular ones I'm familiar with are:
https://github.com/ShikOfTheRa/scarab-osd
and the older/original:
https://github.com/diydrones/MinimOSD-Extra
If I'm not mistaken, I think most minimOSD hardware, if they come flashed with any software, default to the minimosd-extra software, which hasn't been updated in years, but I do know does work (I have 2 APM based aircraft that use this software). I have one minimOSD board setup with the scarab-osd (not the latest version, but pretty recent / within 1 year or so) that's setup for a F3 FC running betaflight (not the latest).
Anyway, as you can see, there are many other options in terms of software and flight controller integration... so, again, more details are needed.