The faster the wood dries, the more movement and chances of warping, cracking and cell collapse.
If you go too long in the microwave it will char or combust.
Walnut can be pretty entertaining in the microwave.
One time I put a block of walnut in the microwave when I was new.
When I took it out there was a small open burl eye with smoke coming out of it.
This had me curious so I took it to the bandsaw and cut it in half.
Inside was a glowing ember like a charcoal.
We use a microwave when making freeform carved bowls.
They carve a fresh/green bowl to almost final size.
Then a short time in the microwave.
This removes a good bit of the moisture but also distorts the shape of the bowl.
(a good thing if you want a random shape)
Then they do the final sanding and finish.