A cut and paste from what I said to SSD:
So it basically breaks down like this:
A lot of people believe that quickening is the primary culprit, but it's not. Being able to permanently maintain a spell is only as useful as the spell itself. 6e fixed most of this problem by removing the defensive applications of armor, astral armor, deflection, prophylaxis, and so on. In 6e the power spells to quicken are increase attribute, increase reflexes, and levitate. Now that is still a very strong retinue of effects, but comparably so to ware and power points for augmentation, so at present it does not have a distinct advantage over other augmentation options aside from versatility (mental attributes).
The primary culprit is dice pool disparity, of which foci are a huge contributor. When the victim's maximum resistance pool is Attribute + Attribute (basically 20 dice at absolute maximum, and only if augmented by a mage since Willpower factors into almost every roll), but the caster's dice pool is Magic (no cap!?) + Skill (13 after max ranks and expertise) + foci (also no cap!?) of course magic is going to be an unstoppable force. You set the math up to be unbeatable from go. The disparity starts off minimal at chargen, and only increases in favor of the mage exponentially the more karma each character has.
I mean, just look at the formulas. If you consider going from Magic 6 to 12 with karma vs. 6 to 12 with a power foci the cost comparison is a joke. Sure, using foci has some downsides, but: 1), those downsides are non-existent vs. mundanes, which is the main balance problem to begin with, and 2), most of the time they are trivial because the God mage does not care about mortal concerns when he can cast his way through very nearly any issue.
The second reason foci are a problem is because not only do they add to your dice pool to make your chosen effect successfully function, they also add to your dice pool to resist the damage you take for making those effects better. Attribute + Attribute (again, 20 being an average cap with elves having a 3 die advantage) + Initiative Level (again, no cap) + foci rating (again, no cap).
By the time a mage has 100 karma, an optimized one will have a spellcasting dice pool and drain resistance dice pool both over 25.
That said, I don't have a problem with foci existing. The concept is cool, and it could be introduced in a balanced way. The bottom line is that magic, both the attribute and foci, need a maximum potential cap for mortal PCs. If you want to have dragons and immortal elves go higher, sure. Attribute cap of 12 and foci cap of 4 (with 4 being absurdly rare artifact level item, and with a dramatic cost increase across the board) would be my recommendation. Even then it still leaves magic at an advantage, but not with unlimited potential.