The addition of the monsters allows the usage of "Union Controller" and "X-Cross Cannon" as extra searchers and combo enablers, and grants access to the "XYZ-Hyper Dragon Cannon" boss monster, as well as easy access to Tribute Summons on the opponent's turn - enabling a board wipe via "Beast King Barbaros"
The most exciting thing is that this ability is given to you during the opponent's turn
They cost one less for each Artifact you control, which is great for avoiding Commander tax, and when they enter the battlefield, you mill yourself for four cards
A deck pairing Arceus ex with Dialga ex has become the new go-to strategy in the larger meta in Pokmon TCG Pocket
When combined with Dawns ability to transfer energy, you can repeatedly trigger Darkrais ability, stacking damage in a single turn
Its always a hoot to resolve, and Im a big proponent of injecting more randomness into your games of Commander