The Happs buttons are made from nylon, but I couldn't find any translucent buttons. I checked a few other sites and they don't seem to mention what they make their buttons out of. GGG has translucents, but they're not made by Happs and it doesn't state what they're made from, I guess you'll have to ask him.
Nylon, even clear nylon, is pretty durable. But I've heard clear nylon is more expensive (I don't know) as opposed to the opaque nylon.
So if the manufacturer decided to go cheap, like say... using transparent styrene, then yeah, it'll be brittle if they don't treat the styrene during the curing process. I don't know if the treatment to make them less brittle makes them less translucent or if it's just a cost issue. Those clear brittle CD jewel cases are made from styrene plastic and I've never seen a clear styrene model part that wasn't brittle.
You'd have to find out what the translucent buttons are made out of.