I've found that such a character is actually possible, though it does make your life a little more difficult as a player, as you need to be constantly thinking.
Let's assume a TeFo Specialist has a Te/Fo 30, allowing them to generate (theoretically) around level 55 spells. That's 2 * 465 XP = 930 XP
Dividing that equally into all 15 Arts, 930 / 15 = 62, which is an 11 in all Arts. Arguably (Sta +2 or +3), someone like that could generate level 25 spells, and being able to learn any spell gives you a lot of options. I'll take the stereotypical Flambeau as the example. CrIg 60, Sta +3. Using Pillum of Flame, his Penetration is at least 43, with no Score in the Penetration Ability (which he would probably have). That's pretty impressive. However, a generalist could easily deal with this, a Fast Cast (or Interrupted, if you're using those rules) PeIg 20 Spell (Base 4, extinguish a fire, cooling the ashes to merely warm, and +2 magnitudes for 2 * +5 damage beyond the first +5 (as per the PeIg guideline, and +2 Voice) would snuff this out in its tracks. PeIg 20 is learnable by our generalist (PeIg 22, Int +3, Magic Theory 5 (Int should probably be lower, Magic Theory should probably be higher)) in two seasons. In a pinch, Spontaneous Casting would generate at least an 11, which could be used to rip 10 damage out of the Pillum, mitigating the amount of damage done. Instead (if the storyguide allows it), the generalist could fast cast a CrAq spell near himself, or in the path of the fire to snuff it out.[sup]1[/sup]. Alternatively, alternatively, he could cast a MuVi 25 spell and change the target to the Flambeau, or a PeVi 25 spell and just write the Pillum out of existence. Of course, the Flambeau might respond with a higher level spell that the generalist can't MuVi (Level 25) or PeVi (Level 35 at least), but then the above point about CrAq applies, or CrTe to create a wall to block the projectile (since the spell wasn't aimed). If he learnt from that mistake, you could always fast cast ReCo and teleport out of the way.
That was a simple example, but the point I'm trying to make is that a generalist has a lot of options, while a specialist has only a few. Sure, leaning towards a favoured Te or Fo is beneficial as it gives you some power to leverage, but a true generalist can still be incredibly powerful, if he's clever. Of course, a generalist will lean heavily on mastering his spells, and knowing how to best leverage them to his use, and he'll spend a lot of time inventing simple utility spells he can master and fast cast like a maniac as the situation dictates (being able to generate 24 tonnes of water in ~1.5 seconds can be awfully destructive in the right hands. CrAq 24 tonnes of water. MuAq it into ether. CrIg to light the ether on fire. Instant siege breaker/house burner/general death machine.)
Remember, when all you have is a hammer, every problem starts to look like a nail. The generalist has many more options to deal with his issues than a specialist, even if his raw magical power is lower.
1 - an Individual of water is a puddle 15ft across, and 2 paces deep at its centre. Assuming 1 pace ~ 1 metre, and puddle is hemi-spherical, 2/3 pi * r^3, or 2/3 * 3 (ish) * 2.3m^3 (7.5ft being just over 2.3m). That gives us 2 * 12.167 = 24.334 m^3. Or about 24 tonnes of water.