Reasons to avoid: Some varieties cause a rash
The many varieties of bonsai tree are delightful, and we humans love anything that comes in mini form, trees being no exception. But there are many varieties of bonsai trees that can cause allergic reactions, and handling some without without protective gear can result in rashes—and can cause some serious toxic reactions if ingested. The danger extends to your pets—if you have a curious critter who might munch on your bonsai, this is a very poor choice for a houseplant even if you plan to be very careful when handling it.
(Of course, the idea that anyone has kept a bonsai tree alive for more than a week, considering the absurd amount of water they need, probably makes this slide moot. Or is that just me?)