Hmm - I hadn't read that birchwood was unsafe - where was that? I know Rodipet houses and things are made from Birch.
The answer seems to be on this thread - some varieties are safe and some varieties are best avoided due to salicylates in the leaves and bark apparently (salicylates are what aspirin is made from)
What wood to use?
I assume Rodipet know what they are doing. The warning is more for people using wood they have collected, or for making things I think.