Unfortunately, the power boost that non-sayians get does not last when they do their one-shot transformation. All characters gain power levels when they attack, but when a Sayian gains power levels, what power levels he gains get doubled when he goes SSJ. For example:
Let's say Goku has a power level of 3 million
Let's also say that normal Krillian also has a power level of 3 million
Goku goes SSJ, doubling his power level to 6 million while he is SSJ, and that power level gets cut back in half to 3 million when he decends back to normal.
Krillian goes Mystic, increasing his power level permanently to 6 million.
Okay, now Goku fights and gains 100 power levels as normal Goku. That makes is normal power level 3,000,100.
Krillian also gains 100 power levels, but he is Mystic Krillian. His power level increases to 6,000,100.
But Goku then goes SSJ, his power level doubles to 6,000,200. Goku and Krillian keep fighting. Let's say each round they gain 100 power levels. They fight for 12 rounds.
Round 1:
Goku Normal: 3,000,000
Goku SSJ: 6,000,000
Mystic Krillian: 6,000,000
Unfortunately, the power boost that non-sayians get does not last when they do their one-shot transformation. All characters gain power levels when they attack, but when a Sayian gains power levels, what power levels he gains get doubled when he goes SSJ. For example:
Let's say Goku has a power level of 3 million
Let's also say that normal Krillian also has a power level of 3 million
Goku goes SSJ, doubling his power level to 6 million while he is SSJ, and that power level gets cut back in half to 3 million when he decends back to normal.
Krillian goes Mystic, increasing his power level permanently to 6 million.
Okay, now Goku fights and gains 100 power levels as normal Goku. That makes is normal power level 3,000,100.
Krillian also gains 100 power levels, but he is Mystic Krillian. His power level increases to 6,000,100.
But Goku then goes SSJ, his power level doubles to 6,000,200. Goku and Krillian keep fighting. Let's say each round they gain 100 power levels. They fight for 12 rounds.
Starting Power Levels:
Goku Normal: 3,000,000
Goku SSJ: 6,000,000
Mystic Krillian: 6,000,000
Round 1:
Goku normal: 3,000,100
Goku SSJ: 6,000,200
Mystic Krillian: 6,000,200
Round 2:
Goku Normal: 3,000,300
Goku SSJ: 6,000,600
Mystic Krillian: 6,000,200
Round 3:
Goku Normal: 3,000,400
Goku SSJ: 6,000,800
Mystic Krillian: 6,000,300
Round 4:
Goku Normal: 3,000,500
Goku SSJ: 6,001,000
Mystic Krillian: 6,000,400
Round 5:
Goku Normal: 3,000,600
Goku SSJ: 6,001,200
Mystic Krillian: 6,000,500
Round 6:
Goku Normal: 3,000,700
Goku SSJ: 6,001,400
Mystic Krillian: 6,000,600
Round 7:
Goku Normal: 3,000,800
Goku SSJ: 6,001,600
Mystic Krillian: 6,000,700
Round 8:
Goku Normal: 3,000,900
Goku SSJ: 6,001,800
Mystic Krillian: 6,000,800
Round 9:
Goku Normal: 3,001,000
Goku SSJ: 6,002,000
Mystic Krillian: 6,000,800
Round 10:
Goku Normal: 3,001,100
Goku SSJ: 6,002,200
Mystic Krillian: 6,000,800
Now, we continue this 10 more times, at the same pace. Each round they fight, they gain 100 power levels.
Round 20:
Goku Normal: 3,002,100
Goku SSJ: 6,004,200
Mystic Krillian: 6,002,800
As we can see, Goku as a Super Sayian, is getting twice as much power than Krillian, because his power level doubled only ONCE when he went Mystic, while Goku's Power level is re-doubled everytime he goes SSJ. SSJ Goku eventually will become much more powerful than Mystic Krillian as the game goes on for quite a long time.
Round 100:
Goku Normal: 3,010,000
Goku SSJ: 6,020,000
Mystic Krillian: 6.010,000
At round 20, SSJ Goku is 1.0002332 times more powerful than Mystic Krillian. But at round 100, after each character gaining a total of 10,000 power levels, the difference becomes larger. At round 100, SSJ Goku is 1.0016638 times more powerful than Mystic Krillian. Now imagine how much this increases over a longer game, with power levels going even higher, gaining more power levels each round. SSJ Goku can become much more powerful than Mystic Krillian.
That is the disadvantage that non-sayian characters have that can only transform once. Their power levels only double once, when they transform that first time. Then all the power levels they gain are counted once, while SSJ's power levels get recalculated and doubled everytime they transform. The power levels get get while fighting are counted once as normal, and gets doubled while they are SSJ, so, fighting as SSJ, they gain TWICE the amount of Power Levels. Disadvantage? I think so.