Ebonmaw covers the details of the Ascension problem, so ...
It's highly likely Syndra simply has no degradation when interacting with magic, I think. The more power a living body accumulates, the more it is affected by the power (via transformative properties, either towards self-destruction or forced change). Xerath himself got his power by interfering with the Ascension ritual in Shurima. Ryze underwent a type of training/ritualistic process that stitched him closer to pure magic. Veigar turns the concept on its head by having magic destroy itself (ala corrupter stereotype). So on, so forth.
Syndra is wholly unique in that her power is entirely natural and unaffecting her mental state. She came to match beings of incredible power, often bordering godlike, simply through training herself and realizing her own inner potential. Of all the 'pure' humans in the setting to date, she is by far the one closest attuned to magic without any negative side affects or 'great prices'* to pay for it.
Where this will ultimately put her is a matter of pure speculation. One could say she might eventually become a god proper, but we don't know what the rules or conditions of a god are in the League setting. That's some pretty far off into the future stuff, really.
*Excluding the whole debacle with the Old Monk and Ionia. That was a result of her pre-existing power more than anything.