FX-wise, they’re very different.
Digitone has, as others have mentioned, its effects on the ‘master’ channel. You can send to them, but the delay and reverb and chorus settings are fixed per pattern. The FX track can be sequenced via MIDI CCs (and thus you can use one of the MIDI tracks and loop-back to do any FX sequencing). The reverb on Digitone is akin to the reverb on the A4: it can be very long/infinite and very pretty. The main per-track effect on Digitone is overdrive.
Monomachine, on the other hand, has per-track delay. This can be used as a sound design option for things like tuned delay / karplus strong kind of synthesis. Other per-track effects are sample rate reduction and single-param EQ. All of which can be used for sound design, and can be p-locked. The delay on the Monomachine is seriously fun.
Other effects on Monomachine have to be used as dedicated tracks, sacrificing one of the six voices for effects. Reverb and Chorus are this way. There’s also flanger, compressor, and a few other effects. The Reverb on Monomachine is based on the ‘Gate Box’ reverb from Machinedrum. It’s got a pretty short decay time and is not up to the kind of long/infinite reverbs of the A4 or Digitone. Gate box reverb can be a fun sound design tool, but if you want longer/spacier reverbs you will need to do external effects (which can include running Monomachine through A4 or Digitone).