Extremely generic as some is smooth and some like oak, way too rough. for the most part there would not be a problem if the wood was not extremely flammable or filled with some preservative, much like the wood,
However the wood slices commonly sold are basswood and only have the bark around the outside so should be no problem when under a half inch thick, but as noted would need to have support without the crumb tray if thicker.
Those who make them do not consider the thickness or even precision 0 degrees angle between the faces to be of much significance, so there is some variety of results.
“crumb tray” alone or “no crumb tray” or "supports’ especially in the free design area will get lots of info there.