I'm not experienced at all with the current portastudios, so I cannot comment on how the mastering functions on them really operate. But in theory there would be no need for further compression and/or EQ, if your listening environment is the same. But one could argue there is more to mastering than that: preparation for streaming, CD-markers if it's becoming a CD or other work if it goes to another carrier, crossfades between different songs, making songs sound as a coherent piece of work together etc.
I'm always hesitant to apply limiting to individual tracks, but EQ and compression (and send effects) can be very useful or necessary. I want the mix to breathe, meaning there should be lots of dynamics in the mix process - if every individual track is at the max dynamically there is hardly any room for mixing, IMO. And I've seen mixes where people first max out every individual track and then need to use extensive volume automation during the mix to make things breathe. To me that feels like twice the work with a lesser result. That's also why I see (and promote) mastering separate from mixing - a dynamic mix leaves you with more room to play in mastering.