It’s a lot more powerful CPU that axoloti, which has a single core
the MM 2 cores for dsp, and 2nd mcu dedicated to io.
I’ve never owned a G2 (only used a few times) , and it used a proprietary chipset, which was powerful at the time, but things have come on quite a bit… so I doubt raw power is close to the MM.
however, at the end of the day, it doesn’t come down to specs.
any of these (including MM) can do poly synths and fx at the same time.
but all of the platforms are limited compare to your desktop, so we patch according to capabilities.
a good example would be reverbs, lots of algos for reverbs, so you’d use a much lighter one on an axoloti / g2, than say the MM (or your desktop).
this also highlights an approach , when you say…
”Can it run several poly synth + drum machine, and FX processing at the same time?”
yes, but not in the same way your (modern) laptop can, i.e. where you don’t even need to think about it.
if you want to do many (heavy) things at once, you may have to choose ‘lighter’ dsp modules, you as the patcher, get to choose those tradeoffs. this becomes esp important in polyphonics setups.
Sounds like you are a G2 user?
if so, Id say main difference you will feel is not so much processing power, you’ll have lots - but rather the control surface, the G2 has a lot of encoders 
Axoloti, is kind of different, really its more aimed at DIY enthusiasts, kind of a musical Arduino.
so main advantage is it was easy to create your own instrument, by soldering pots/buttons and encoders to it… also it has incredibly low latency, again aimed at instrument bulding.
the modern equivalent would be daisy. though that needs a bit more coding experience.
all have their advantages, and there is no doubt… the G2/Axoloti modules were highly optimised to their hardware, so what they achieved with the limitations is amazin.