Let's take a moment to look at 2 Bux cards, Plasma Attack and Mystic Might.
Plasma Attack
Upkeep: 500 Bux each
Level 1:
Att: 70 Def: 25 Move: 15
Level 2:
Att: 88 Def: 32 Move: 19
Level 3:
Att: 105 Def: 38 Move: 23
Level 4:
Att: 123 Def: 44 Move: 27
Level 5:
Att: 140 Def: 50 Move: 30
Mystic Might
Upkeep: 48 Bux each
Level 1:
Att: 55 Def: 22 Move: 15
Level 2:
Att: 69 Def: 28 Move: 19
Level 3:
Att: 83 Def: 33 Move: 23
Level 4:
Att: 97 Def: 39 Move: 27
Level 5:
Att: 110 Def: 44 Move: 30
Because there is little excuse not to tier your Bux cards, we'll work with just the level 5 tier on these cards.
Att: 140 Def: 50 Move: 30 @ 500 Bux Upkeep
vs
Att: 110 Def: 44 Move: 30 @ 48 Bux Upkeep
500 (Bux Upkeep)/48 (Bux Upkeep) = 10.4
So for the same upkeep cost as 1 Plasma Attack, you can get just under 10 and a half Mystic Might. Let's do the math and see what this means.
10.4 x 110 = 1,144 Att
10.4 x 44 = 457 Def
10.4 x 30 = 312 Move
So, for the same upkeep cost, you get 1,144 vs 140 for Attack, 457 vs 50 for Defense, and 312 vs 30 for Move. That's a huge difference in points for the upkeep cost. So there are very few reasons anyone should be considering the Plasma Attack card for their deck.
Now, you might think that your deck is already full and so you don't have to fill that many spaces, making this a viable option. That really depends on what cards are in your deck.
Let's say that your Attack card deck has 100 untiered Cupid's Bows as the weakest attack card.
Att: 52 Def: 25 Move: 30
Let's also say we have only 1 Plasma Attack tier 5.
Att: 140 Def: 50 Move: 30
If we replace 1 Plasma Attack with 10 Mystic Mights, we are also replacing 9 of the untiered Cupid's Bows.
Now, 10 Mystic Mights come out to
10 x 110 = 1,100 Att
10 x 44 = 440 Def
10 x 30 = 300 Move
9 Cupid's Bows come out to
52 x 9 = 511 Att
25 x 9 = 241 Def
30 x 9 = 291 Move
Plus the single tiered Plasma Attack
Att: 140 Def: 50 Move: 30
511 + 140 = 651 Att
241 + 50 = 291 Def
291 + 30 = 321 Move
Now, let's compare the totals of those 10 cards to that of 10 Mystic Might.
1,100 Att vs 651 Att
440 Def vs 291 Def
300 Move vs 321 Move
See the difference? You lose a little move, but gain huge sums in the other two stats. And one might argue that it does give 21 more Move, but that doesn't make up for 449 less Attack and 149 less Defense.
The ONLY reason you should ever consider this card for your deck is because a tier 5 Mystic Might or better is your lowest card in your Attack deck and you have Upkeep room to spare, or you are running a deficit for something like LWs and your bankroll can handle it.