I’m keen to know how much better the experience is than using a MiSTer FPGA. I love my Analogue 3D and it’s definitely a better experience for N64, but for the 16-bit consoles I’d love to hear experiences.
From what I understood the older MiSTer core wasn't amazing accuracy-wise but it did the job, the newer MiSTer core is pretty good. I think it was mainly the sound that was an issue with the older ones.
Personally, I have always preferred playing on the Mega SG, it looks nice in any set up as well. It isn't without issues though, accuracy is definitely not 100%. You can only play Mega CD games with the expensive Everdrive/Terraonion cartridges, it is also a faff (and expensive) if you want to play 32x games, the latest firmware doesn't work with the 32x or Paprium. So you have a choice between quite a few fixes and other games.
What I will say. Anyone that knows the Mega Drive / Genesis in general will know the different revisions have their own set of issues / advantages and disadvantages. It isn't exactly the perfect system anyway

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I'm not sure on the NT, but like the SG it looks nice and plays really well from what I can tell.
The older Analogue consoles do not have much in terms of enhancements, basic scanlines / smoothing etc. The 3D is really special.
If you already have a MiSTer, it isn't going to do anything more than what you have, just a bit different (if that makes sense). Also, the MiSTer has some nicer scanline options and can output slightly higher resolutions as well.
I am intrigued to see what additional features the + systems may add.