Well they exist because the archetype of a mystical warrior is a fantasy staple. Ninja, The Witcher, D&D rangers, paladins, and warlocks, take your pick.
They aren't better than an optimized mage (In this hypothetical soak tank/defense tank build a mage with a power foci granting 3 extra dice is rocking 4 extra defense dice, 1 extra soak dice, .5d6+2 more initiative, an extra 3 dice to cast spells, +2 dice on offensive actions, and this is if they decide not to also boost reflexes, strength, and body for an extra 1 defense dice and an extra 2 soak dice in a quickening build and the interaction between edge and quickening that makes 3 dice in reality some odd 5 dice which boosts everything further and puts the mage above 6 PP, which comes out to 4 PP ignoring the casting pool bonus and when the mage isn't trying very hard. Yes, in theory a mysad tank build can go higher than an extra 3 dice to resist damage at gen, but only by 3, and the difference between 40 soak and 43 is rather immaterial when the mage is also doing a heck of a lot of other things better.
But while this is a good mechanical comparison, its easier for a GM to spot that mage and say 'no' and its harder for a player to stumble upon the fact the mage with a power focus is so strong if they just buff like crazy and take psyche or view buffs as a consumable they will lose once every few runs (if ever, its REALLY hard to lose high force quickened spells, like dragons struggle to counterspell those suckers) and just quicken. So while a mage can be stronger than a mysad and is just generally a better more versatile build, it is easier to break mysads than mages.
But what about a 'fairly' played mysad? The guy who is getting some increased reflexes, 1-2 ranks of improved ability (blades), and maybe a rank in combat sense? Those seem to be ok. They are better than adepts just because summoning, but summoning was way too strong and nothing with summoning can be fair until summoning itself gets nerfed, and adepts are pretty bad in 5e and desperately need to augment with secondary power sources like 'ware or drugs anyway, so balancing 'down' to adepts doesn't make sense. In that scenario it seems to me what makes that mysad 'fair' even in 5e where they merely pay a bit of starting karma is the fact they aren't pushing the PP limit that hard. That may be a key to balancing mysads, capping the limit on bonus dice from their powers much lower than an adept, rather than removing magery for adeptness, because doing that doesn't really encourage the "Paladin who fights good with a sword and can also heal" so much as divides resources too much. In essence, a limit on how much you can invest adept powers to particular dice (say... only +1/3rd your magic as opposed to +your whole magic score to any given dice roll like most adepts have) might encourage mystical ninjas and the like and stop buff stacking. With buffs getting weaker in 6e that means the mystic adept can't as easily just buff themselves past a normal adept as well.
Course this is all navel gazing not super related to 6e. I just really enjoy the concept of myasds (They get people into the finger wiggler side of magic might otherwise not be) even though I don't like their implementation.
