Objection! Dialga Lv. X is playable! So are most of those, actually...
Luck-based cards are not automatically bad cards. So you flip tails sometimes. You flip heads sometimes too. What about Riolu? It was considered a better starter in Mario than Machop, and their attacks were similar. Riolu required a coin flip, and did more damage because of that. Machop did more than half as much damage, but did not require the flip. On average, Machop would do more damage, so why was Riolu considered to be better if coin flips are so terrible?
Lv. Xs could use a few more restrictions IMO, but have several more than EXs built in.
The difference is that, at the time, Riolu did a better job of countering opposing starters than Machop. Riolu OHKOed Castform 50% of the time, while Machop 2HKOed it all the time. Plus, if you wiff the flip, you just evolve to Lucario next turn and kill it that way.
In contrast, there's no real reason to use Dialga in a serious deck for the same reason people hate Porygon Z (a much more feasible comparison): all the card does is flip, and there's a 1/4 chance of getting the desired effect. Furthermore, there's no skill in hoping your opponent flips coins as to miss their entire next turn. If you were to win a tournament because of making people skip turns, you would do so by dumb luck - not skill.
A certain amount of coin-flipping is acceptable. If you expect that people will run a 50 HP fighting-weak starter, and you know of a deck which can kill that thing T1 half the time (nearly guaranteed kill on T2) while still doing good against most of the other expected decks, playing said deck is certainly a viable option. Playing it under this set of circumstances can be considered a "smart play."
Again, though, Dialga X is not a Metagame counter like Riolu was. It has just as much of a chance to skip the rest of your turn as it does to skip your opponent's entire turn. No matter which player's turn is skipped, you do nothing but make yourself look foolish by even trying such a stupid stunt - and as stated earlier, people will attribute any victory to pure luck, not your skill.
That, in a very poorly organized nutshell, is (IMO) why Chuck stated that Dialga X is bad. It makes the player look like an idiot (again, IMO).
