there are reviews like now by BLAWAN ( great producer, i really like his kind of techno ! )
that might give you the impression, the built in FX are, well, "mediocre".
From my side i can only say this: the first time i tried out the MB in a shop, i dialed in some effect and it seemed to
make the sound washy, blurry, take off too much of the original signal. so i thought, well, ok, nice add on, nothing too fancy...
But, after several weeks now with the MB, i do not only appreciate ARTURIA's decision NOT to use some digital FX but BBD based analog ones,
but - and this is the IMPORTANT part: so many of my patches would be IMPOSSIBLE without exaclty these FX.
You have to try out and modulate and they are not FX like a set and foget digital reverb, but instead with investing time, you get great results,
from doubling, vibrato like sounds, to very unique spiced up patches, you get breathing on pads, dirt on your bassline and much much more.
Modulate it's parameters, make fine adjustments, find the right spots, all that will be rewarded!
To me, these FX make A HUGE impact on the overall unique sound of the MB.
i can only speak for myself, but don't say "nah" to these FX to fast. i think you would miss a good part of the MBs potential.