They're far from the real deal. Nub on the directional pad center isn't the same. Buttons triggers feel different and less responsive. Plenty of in depth breakdowns out there that show this. The 2nd gen originals (ones without the engraved logo), are unmatched.
Same company did the switch pro controller i believe and the d-pad there is lacking as well.
They are still high quality, and close enough of a replica that someone who hasn't held a controller in 20 years won't know the difference. I hardly notice any difference in feel during actual gameplay, only that yeah, if I hold controller A and controller B in my hand, I may notice some slight difference in feel. The internals are changed ever so slightly that they are not drop in replacement parts for the vintage controllers. However given the scarcity of these things and the fact that many casuals who pick up Mini consoles probably don't have original hardware laying around, I just don't see a market for classic controller to original console adapters.
You can buy ten packs of silicone membranes for NES and SNES controllers on ebay all day long for cheap. 99% of the time, a deep clean and replacement of worn membranes will make any crusty old worn out controller feel factory fresh.
And lastly, the Dpad on my Switch Pro controller is flawless. I don't see what the effing deal is with the complaints. It's by far the most comfortable modern controller I've ever held.