Planers like to chip out pieces on cross grain, and I would think that a log would have some issues.
I think that the bark might give a planer issues. Between tearing off too much bark and dirt messing with the knives.
The size of the planer might be a concern. I have a small one, I think it is 6 or 8" wide, but the distance between the rollers is like 8"+. With regluar lumber you have things that are long and narrow, a log slice is short and wide. The slice may not even fit.
I really enjoy cutting log slices, they end up looking really cool. I use a belt sander to get them smooth and to the right thickness.