Basically what NZA said. David Sirlin (the guy who balanced HD Remix)
actually wrote four articles five years ago addressing the same issue, and they're good reads. The more asymmetrical your game is in the starting options, the more you need to take fairness into account and offer more viable strategies or safeguards for players.
Let's take Guilty Gear as an example, quite possibly the greatest 2D fighter ever made aside from Street Fighter, and one of the most balanced fighters I have ever played. GG is extremely asymmetrical, with an incredibly diverse set of characters that each play radically different and have traits that seem overpowered on paper. Zato fights using his shadow Eddie, and you're essentially controlling two characters at once, coordinating their attacks to work in tandem with each other; Venom can place multiple pool balls on screen in various formations and hit them off each other, creating various traps and set ups for approach or zoning; Zappa sucks on his own, but can summon 1 of 3 spirits at random, changing his playstyle and moves with each one, collecting souls so he can summon his ultimate spirit Raoh, transforming him to a borderline broken character; Baiken has guard cancels which work like alpha counters, allowing her to attack during blockstun without meter, etc. In order to keep all these crazy characters in check though, various system mechanics were implemented, though I'll just name only a few for examples. This is why Arc System Works is on a whole different level from everyone else.
- Burst: Simple. All characters have access to a burst that they can use at anytime. The burst meter keeps track of how often you're allowed to burst. You always start the match with a full burst. Once you use it, you can't use it again until the burst meter is filled, and it fills slowly the more you get hit, with some moves filling it faster than others. If you burst while being comboed, you knock the opponent back and break out the combo. If you burst when you're in a neutral state (not blocking or getting hit) and the opponent gets hit by it, they get knocked back and you're rewarded with a full super (tension) meter. This adds to the strategy of when you want to use it, as good opponents can easily bait a predictable burst. Do I want to use my burst now, or save it for later? OR should I take a risk and try a gold burst to get full meter, at the cost of probably eating a nasty combo I can't break out of later on? Decisions, options, players must be given these.
- Faultless Defense: You can perform faultless defense by blocking and holding two attack buttons. During FD, you take no chip damage and the opponent gets pushed back further after each hit you block. Doing this slowly drains your tension meter. So as long as you have meter, opponents can't simply chip you to death or lock you down for the win.
- Guard Meter: Another safeguard against infinites, as well as punishing overly-defensive players. The guard meter starts 50% full, and wanders back to 50% overtime if it gets raised or lowered. The more you block attacks, the higher the meter gets, and the less you benefit from damage scaling. So if the opponent does eventually penetrate your defense, their combo will do more damage than normal. This is why meter is so important, as FD is much more useful defensively than blocking normally, as it doesn't raise the guard meter. On the flip side, the more you get hit, the lower the guard meter gets, and the more you benefit from damage scaling and the less hitstun you receive. So even if an infinite combo did exist, it wouldn't kill you because eventually the combo fail and you would drop out.
- Negative Penalty: If a player is constantly running away (backdashing, airdashing away, walking backward) avoiding any type of contact and isn't performing any offensive moves for a prolonged period of time, a warning will appear on the tension gauge. If the player keeps running away, they are given a Negative Penalty. Their entire tension meter is depleted, and their meter gain is at 20% than normal for 10 seconds. If you're playing zoning or keep away, the game will not penalize you, as you're still performing offensive actions.
- Tension: Incredibly important, tension is needed for offensive and defensive purposes. The only way to get tension is by performing any type of offensive action, whether it be attacking or advancing towards your opponent. Heck, even just walking forward builds meter. This forces people to go on the offensive more than defensive, and helps make matches more interesting.
And that's just a few. With just a few of these system mechanics, it helps massively in keeping the cast in check and goes a long way towards making the game balanced. Since these mechanics are shared by all characters, EVERY character has multiple options for any situation. With all these safeguards, ArcSys has the freedom to go truly crazy with their character designs. Because they know that system is there to back the characters up and help them get out of trouble if things get too crazy. That's brilliant game design.