The problem with the Jirachi is the 90 HP. Darkrai needs a muscle band to KO Shaymin, which is usually spent on Yveltal or Zoroark or whatever attackers Tool Box may bring. Jirachi tends to be a free 4-prize turn if you previously failed to KO another EX. 110HP in general often needs that little bit extra that spread-damage moves can't always give. I've experienced this both ways often enough to disagree with your conclusions.
You just got really, really specific. If this was supposed to be "...on top of everything else, what about this?" okay. I read it a bit differently though: if I may coin a phrase, it seemed like you thought this was a "silver bullet point" argument that would put my side down and keep it down. XD
Yes it is true;
Jirachi-EX is easier to KO than
Shaymin-EX, barring things like Weakness and Resistance.
Darkrai-EX isn't an attacker in every deck and in the decks where it is, it may have
other means up boosting its damage. For example while grinding out the last of the VS Ladder Rewards (or whatever it is called on the PTCGO) I had a
Shaymin-EX KOed through
Hypnotoxic Laser +
Virbank City Gym + Night Spear. I also more than once used or faced someone using a
Darkrai-EX that had
Muscle Band attached. The fact that 90 HP is even more vulnerable than 110 HP isn't in significant, but it also isn't huge. Your opponent can get a "four Prize turn" in more than one way... but the method you listed required that he or she
failed to score a OHKO on another turn. If my opponent scores 4 Prizes on a single turn instead of two Prizes per turn for two turns, it is only in some messy details (like drawing more off of an
N than they otherwise would have) that proves relevant.
100+ HP tends to also be some sort of mental barrier for people playing Bats to put damage on these Pokémon, too. And finally, unless you're running Archie/Maxie, searching a supporter isn't quite as powerful as drawing cards and playing the supporter you're going to draw 8 times out of 10. Especially with VS Seeker, which combos with Compressor to make Jirachi almost entirely obsolete. I run a "Battle Seeker" engine in just about every deck I play.
If people have some sort of "mental barrier" about putting damage counters on something with 100+ HP, that's a problem with those players. It isn't "general wisdom" when playing the deck. You evaluate what is most likely to help you win the game. Usually that means focusing on the Pokémon with the lowest remaining HP.
Battle Compressor +
VS Seeker is a potent combo, but that doesn't mean
Jirachi-EX is obsolete. Different things block them:
Jirachi-EX can still get you the Supporter you want even when Items are being locked down, though of course does nothing when Abilities are being negated.
VS Seeker is much more difficult to search from your deck than
Jirachi-EX; if Items are working, then
Level Ball =>
Jirachi-EX => Supporter of your choice. If your deck can't justify
Level Ball, then replace with "
Ultra Ball + two card discard from hand". Getting into fringe territory (meaning I know this does not apply often), sometimes you will need all the uses of a particular card that you can get. The
big thing is that
Battle Compressor +
VS Seeker is a minimum three card combo to get the Supporter you want from deck to hand, while
Jirachi-EX is just two cards (yes the Supporter being searched out counts). If I don't already have
Battle Compressor or
VS Seeker in hand, I can't even hope to draw the other on my next turn... or I can, but that probably means I just used my Supporter for the turn so my plans are moved a turn ahead.