I'd like to know which of those you find unbalanced Psion...
First of all, remember... balance is in relation to the setting only. So as long as the magic system exists standalone, it's not unbalanced, as I see it. You may think it's over/under powered compared to normal DnD magic, but that's not relevant in the overall comparison.
Most of the listed ones I would actualy say are less powerfull overall than normal DnD... Chaos magic has raw power but it's too darned deadly to use... Sov Stone power is a _little_ more powerfull in the begining, but not as levels increase. I have no experience with the Deadlands magic, though, so I can't comment there... Wheel of Time may have some, but again, only when compared to normal DnD magic.
As for "why"... I could as easily ask you "why not". It's not so much a predjudice against official sources -that makes it sound like an evil thing-... More, after years of Vancian magic, I'm open to alternatives just to spice things up. And sorcerer magic is still Vancian, sorry... It's just a little more freeform. Heck, Wheel of Time is still based on vancian magic at it's core.
As for some brief descriptions...
Mongoose's Chaos Magic allows a user to "sculpt" spells on the fly by picking different components to the spell off a sort of "menu", then they try to cast the spell. But it's very deadly... not only is killing yourself through outright spellcasting possible, but you accumulated bad effects when you fail spellcasting roles.
Wheel of Time magic is sorta like a modified Vancian magic... At least, it uses spellslots. You learn "talents" (sorta like schools) of power, which you can use to cast spells. Each talent has different types of spells assosiated with it, and by using different power slots when casting the spell you can create different effects.
Sov Stone magic is elemental... You have four elements (Earth, Air, Fire Water) four combined elements (Nature, etc... elements formed of two of the prime elements) and void, the "fifth element" that doesn't mix with the others. Each element has different spells assosiated with it (Similar to Wheel of Time and talents), but they are more traditional spells... Many of them are just normal DnD spells, with the level replaced with a casting threshold. When you try to cast a spell, you roll a d20 to make the casting threshold. If you succeed, you cast the spell. If not, you keep going. Failures cost fatigue (represented through subdual damage), but there is not "hard and fast" casting limits... a 1st level elemental wizard could theoreticly cast any spell a 20th could for the most part... if he could survive casting it, that is... and a high level wizard can cast low level stuff all day because he will almost never take damage from the low casting threshold on the lowerlevel stuff. Void magic (sorta like necromancy) has some pretty powerful spells, but has downsides to casting... You take real damage, not subdual, and magic healing doesn't heal you as long as you have "void taint" (damage from casting void spells). Each race (in sov stone... in normal DnD you could just allow a person to choose it, I suppose) has a "favored" element... and an element they are weaker in. In sov stone, for example, elves are favored in air, dwarves in fire, ork in water, etc. Sov stone magic is by far and away my favorite d20 magic system, even over official magic systems.
I havn't got enough chance to look over Fading Suns to give a GOOD description yet, and I have no expeirience with Deadlands.