A lot of the original nitrate negatives are lost anyway and have been long since transferred onto 35mm 'safety' film so they are 2nd gen dupes. So I think if a new transfer is made from the original nitrates it would look stunning because that film was known for the quality of the picture, but otherwise you'll get lesser results, still good, but not AS good
I think a lot of 80s films are going to look the worst when remastered to 4k, the film stock in the 80s is well known for being cheap and low quality. This is especially bad in Asian films where they didn't use Kodak stock, I think it was Fujifilm which is why they have that weird greeny yellow look to them.
So yeah, there are a lot of reasons why a 4k disc could look awesome or not.
There is also the disc production as well, the actual mastering of the transfer, some companies do very strange things to their films to make them look more modern. Some directors as well, Peter Jackson and James Cameron like the clean modern look and try to erase the film grain that digital doesn't have. Madness!