At 280 HP and with straightforward costs for its attacks, Umbreon ex SIR is a flexible threat that earns a spot in slower control strategies or as a closer in prize race decks
And while the energy cost is a little higher, you should have both of the energy cards attached once you are able to evolve him, as you have a chance to put a bolt on Voltorb first
We see this all the time, regardless of format: White Weenie aggro decks in Standard across time, and most notably Affinity and Zoo decks in current Modern
I can see getting out 3 Lightning Chidoris from a single activation, getting rid of like 6 cards from the opponent's field, then using the Chidoris for a Link to free up space for a second Harpie's Copycat, reviving 2 Harpie's Pet Baby Dragon (potentially destroying 2 more cards) and 3 Harpie Lady 1 copies
The Amber Amethyst Bruno Madrigal-Tiana deck relies on singing songs to gain lore and to weaken your opponents characters in play