There are two reasons why I don't like EX: Emerald:
1. Steelix ex.
2. The Pokémon-ex have ridiculously low HP.
Steelix ex is one of the most dangerous cards in the format, with the ability to give 100 damage to anything. For this reason, I tend to give prejudice towards monsters with 100 or less HP.
Excluding Pokémon-ex, there are only three Pokémon in EX: Emerald that will survive Mudslide: Blaziken, Exploud, and Swampert. Blaziken and Swampy survive with 10 HP; Exploud survives with 20.
The Pokémon-ex don't do much better at surviving Mudslide, take a look:
Milotic ex has the highest HP of all the Pokémon-ex in Emerald, with 130. This is actually good; after a Mudslide, you're alive, which is good, and you're not in extreme danger, which is better. Unforunately, Milotic ex is more a tech than an attacker. However, the fact that it can live through Mudslide and do something about it makes it one of the only decent Pokémon-ex in this set.
Camerupt ex has 120 HP; kinda low, but still acceptable. It survives Mudslide, but it's in more danger because lots of things can give 20 damage to the Bench. Granted, most won't be played with Steelix ex, but the possibility is there. Then again... Steelix ex is weak to Fire, so if one shows up, get out there and Eruption for 120 (OHKO if either one of you gets an Energy discard).
Here's where we run into trouble.
Regirock ex, Cacturne ex, Deoxys ex, and Medicham ex all have 110 HP. That many HP on a Pokémon-ex is inexcusable; it's almost like you're begging to have them sniped. All they need is a single damage counter and they're Mudslide bait. It would be better if the cards were powerful, but alas, that isn't really the case here, with the exception of Medicham ex and possibly Deoxys ex. They are susceptible to Desert Ruins, too, just barely above the trigger point. Gross.
Raichu ex, Regice ex, Altaria ex, and Dusclops ex all have 100 HP. This is like a slap in the face to all Pokémon-ex now that Steelix ex is on the prowl. That single damage counter that saved the others is no longer needed here; Mudslide will OHKO them, and there's nothing any of them can do about it. Two free Prizes, coming up. These are also susceptible to Desert Ruins, having the bare minimum HP needed to trigger it. This is bad.
Registeel ex has 90 HP. It's Metal, so it gets Metal bonus. It'll live through Mudslide if it has two Metal Energy attached to it. It's immune to Desert Ruins, but the fact that it's a Pokémon-ex with 90 HP means it's nothing but trade bait. Registeel ex sucks anyway; if you use it, you deserve to lose.
That's my two cents on EX: Emerald.