Its not a computer, its a small device. You dont have many unknown peripheral you dont have other programs. The memory and peripherals are there, just use them. Heap is complicated ? Preallocate everything. A peripheral is not used ? Just leave it there. Security ? Of what ? Thats the appeal of those devices.
Your comment got downvoted but I think there's deep truth in it. I've been decompiling GBA games from my childhood and it's remarkable how engineless they seem to be.