But NFS Rivals did look a bit better on the PS4 though -- a few more visual effects.
Overall, this is a large open world game so it not looking "that great" in visuals isn't too surprising to me. On top of that, the decision for 30 FPS for both versions could have simply been due to frame rate dips in the game which would somewhat make sense considering it seems like there's a lot of action going on in the game's huge open world.
Anyway, not every multiplat game will have the same difference between consoles as the multiplat(s) with the biggest differences between each version. This was true for every gen. There will also be a few cases in which the version on the weaker console will look and/or play better -- something that was also true for every gen including this one.
There will be more games with this multiplatform difference (or lack thereof) in the future. Deciding not to get those future games for the console you own based solely for that reason will more than likely mean missing out on a decent number of solid multiplatform titles this gen. If people want to do that then that's them but to me that doesn't really sound enjoyable, especially if the games are solid either way. I don't remember people taking this "stand" in previous gens so I don't really know why things are different this gen unless I'm not remembering things correctly when it came to those previous gens. Might be that... ::shoulder shrug::