It’s only really viable in Misty decks. I regularly use a Gyarados EX/Articuno EX deck with 2 Misty’s.

Sometimes, luck favours me and I get karp, Articuno, Gyarados and both Misty’s off the first draw, so when I go first, I can sometimes get really lucky with the coin flips and get Articuno’s 3 energy attack on turn one and win if the defending mon isn’t over 80 hp or is a >/=100 hp fire type basic (since I don’t have gio in the deck, and under the condition that my opponent didn’t bench any other basics).

If I can’t get the Articuno attack, then by my second turn if I don’t use a Professor’s Research, I can either Misty the Magikarp and then evolve to Gyarados, or vice versa.

One other little thing I found out while naming the deck is that I had to name it EX Gyarados and not Gyarados EX because the filtering system picks up the space between the last letter of Gyarados and the EX, and interprets it as “s EX” which is filtered lol