Genuine Japanese futon on tatami are not all that comfortable although it is probably one of the best overall solutions to minimalist bedding.
The traditional styled, natural fiber ones seem to get mushed down over time and just get harder and thinner so one decides to sleep on layers of two or three.
I've found the ones with modern foam style padding to be much more comfortable.

They do need to be aired out often. Saturdays is the traditional day to throw one's futon over the balcony railing for airing out.

We do not have a frame for our IKEA double-sized box spring & mattress as it sits on top of tatami mats. A traditional western frame would damage the mats and is way more bed than I want to own.