Yes
I don't necessarily try to fool people into thinking they are "real"pokes because I don't want to use them in tournaments where they are not allowed or trade them to someone who does not know. But i do try to make them base the global battle union legality checks, which just means they have to have the right moves and abilities and meeting place etc. that is hard work some times.
Also hidden powers are tricky too.
Sometimes I make "illegals" like Spiritomb with wonderguard ability. I usally try to name those illegals so people know up front.