Just from what I have seen and heard because I have been watching the subject for years.
They cost quite a bit.
They very often never actually run right.
They often don’t last a long time. (Failure rate)
All 3 of those points are totally valid. I would also add the question of parts availability. e.g. just exactly what throttle position sensor (or any other component) does my aftermarket EFI system use? Can I get it from the auto parts store today, or tomorrow at the latest?
All 3 of those reasons you said plus the one I mentioned is why I used a cheat code to get cheap, perfectly tuned, reliable, and servicable MPFI: EFI Magnum swap. Grab a 5.9, wiring harness, and computer from a 90's Ram (or B van, in my case), and drop it in the old A body using the factory EFI. Yeah, it has its limitations, and there's no instruction manual or tech support you can call, so it's not for everyone. Definitely not for the OP since he already has a decent 340. But it's perfect for me.