Terrain and path interaction could be a lot better in Planet Coaster, I agree with that. There are also some systems from RCT3 that we would expect to be in Planet Coaster because they are kind of 'standard', like synchronized stations or shuttle mode for transport rides, etc. However, keep in mind that PC is not trying to be RCT3 and tries to improve the genre in a different way, namely getting rid of the grid system. The grid makes making paths and terrain really easy but it also removes almost any form of freedom.
PC's path and terrain systems are both very powerful and fairly complex give us a lot of freedom. A possible downside of this is that it can be difficult to get the hang of those systems. I say 'possible downside' because personally I don't mind spending some time to get that hang of those systems. I am not saying the path and terrain systems are anywhere near perfect, but when you actually take the time to learn how those systems behave etc, they aren't nearly as bad as people make them out to be.
The same thing could be said about the coaster builder. RCT3 had pre-made pieces that align with the grid so making a coaster is very easy and quick. In PC we have the spline based system that except for the silly limitations the devs put in place allows us to make the track in any shape we want. It does take more time and is more difficult to use, but again that's the sacrifice you have to make if you want to have a more complex system.
Since I've played Planet Coaster, I cant go back to RCT3. I have tried, but after playing PC the grid system in RCT3 feels so limiting I cannot enjoy the building aspect anymore (I never really played these games for their management so not going to comment on that). So give me the freedom of Planet Coaster any day over the very limited grid system in RCT3 despite it makes 'some' things easier to do.