If something cannot be uncrafted, it is definitely not intentional (as in hard-coded), but in most cases is simply unavoidable. I haven't actually tried recycling anything nuclear-related, but when you say "cant be recycled", do you mean it refuses to accept it, with the "cannot be smelted" message, or do you mean it attempts to be recycled, but fails to output anything and gets "stuck"?
I have noticed a few things have changed in how Factorio works, changing how the mod works, despite little to no changes in the mod. Like the satellite can no longer be uncrafted, despite having enough output slots for it. Also that after recycling one type of item, you can't recycle a different type of item until the output is completely. This was not the case in 0.14, where you could continue recycling differing types of items until the output slots were all full.
I might look into this later