This stage is has its ups and downs. You can use u-airs through the floors and abuse dtilts in this stage. However some characters like Dedede, ROB, Snake, and Metaknight (Just no contest the match for MK lol) will hurt you here, who are mainly the ones that are tough for Marth anyway. The stage does not give much advantage against your weak matchups so it isn't a smart counterpick. In the end, like all stages, it depends on your opponents character.
However, you can live longer by staying in the mansion, your priority will help you get the ko's needed outside of the mansion. Also note that dolphin slash can send opponents at a trajectory in which they will avoid the mansion walls and lead to edgeguarding. You have decent options on the ground, and odd options in the air. What I mean is that a fair will go from the ceiling to the floor and wall opponents completely. A nair will be harder to avoid and dtilt traps are gold.
I rate the stage a 7.5/10. Would be higher, but the stage doesn't help against the characters that counter Marth. Which is what a counterpick is kinda supposed to do.