There's a difference between wood and melamine
Melamine is already coated,wood wouldn't be.The only thing to worry about is sealing the edges as mentioned,as you would do with any sort of melamine cage.Improperly sealed cages are going to have problems with urine absorbing into the melamine,but with proper sealing it isn't an issue.
The only wood involved would be with a lid,and if a hamster somehow urinates on a lid there's something seriously wrong with that hamster who just might be possessed or something!