Yamaha MM6 is something like synth with arranger functions (while PSR E-403 & E-413 are arrangers with synth functions). But at the same time it lacks some synth functions as well as some arranger functions.
There are frequent users complaints on 'motifator' forum, because MM6 is presented as Mini-MO, but lacks some important synth features, like monophonic sound, or portamento. Also, sounds changed by user cannot be saved as sounds, only as performances (along with some other changes that user might not want to change), and performances cannot be renamed.
The sound quality is also not on Motif level (ROM size is 75 MB, while Motif&MO has 175 MB) so there are just couple of Motif sounds, and the rest are PSR sounds.
Arranger users might be limited because there can be only one user-style loaded at the time, and only 32 notes polyphony might give odd sounds in live play.
So it's mix of synth and arranger, with some limitations, but also some advantages, and might be very good choice for home production , as well as live jamming of modern music.