I've read reports of them bending in half and tumbling with no expansion. Some have good expansion like a hunting bullet. Yet others have the jackets and cores separate since they aren't mechanically locked together or bonded and it all becomes shrapnel. They'll kill game, sure, but no way to tell if it'll be a clean kill or not or what the bullet will do. If I had no other options I'd use them before I used a FMJ but if I had access to anything like a real hunting bullet I'd go with that option first.
Illegal would depend on your local laws which change depending on which part of a given province you are in.
For any calibre I know of that HP bi-metal ammo is available for you can find proper hunting bullets without looking too hard. 223 and 308 are everywhere but even 7.62x39 has soft points available. Tula makes a soft point in x39 and I've also seen Federal and Winchester floating around.