Honestly I don't think Isengrim and Elirenn will be OP with this ability, Artefact can be removed and even if they don't +3 points Isengrim and -3 body Aelirenn is far from being the most OP thing you can do with a leader in this game.
Alright, a bit of an exaggeration perhaps
. Really good isn't stretching it though. To throw out an example board state...
Say you have 3 elves on the board and 2 spent traps with the points between both players about even. Now you transform both traps and play Isengrim. Right there you went up 6 points, Isengrim gets 9 value and Aelrinn comes out for 5 value. That's a 20 point play. I wouldn't call that insignificant. There are so many ways you can use that leader ability to get elf tokens to point-slam a board.
Another example would be to flip traps to pull Aelrinn then drop Pitfall on the same turn. If you had a decent lead there Pitfall is going to block the next card play. The opponent may be forced into a pass because of it. It could easily be leveraged into a, "Get the hell out of my round", type of play.
Artifacts can be removed but at the moment most builds are running 0-1 artifact removal. 2 is risky in most cases. It's unreasonable to assume traps won't stick.