[Fluffagrams]

I wouldn't say that they were unsafe as such unless you had a particularly chewy hamster who might chew the plastic to the point that pieces come off but you may find that some tubes are unsuitable for some larger Syrians simply because they just won't fit through them with ease and may get stuck. As Cypher has also pointed out, the ventilation may not be as good as a more natural choice if your hamster decides to make their nest in one.