There are some spells, which have such a high Drain, that they become essentially useless, but the vast majority of spells is pretty easy to cast. Some are actually
too easy to cast (Stun Bolt is the prime example here; a Force 5 Stun Bolt does only 1 DV of Drain; a Force 9 is at a meager 3 DV).
Besides, what's the deal with taking a few boxes of stun damage?
It's not like you are supposed to cast all your spells without ever suffering a single box of damage, if that was the case, there would be no point for a Drain rule in the first place.

High Force spells are very powerful, there needs to be a downside to them. You can always cast spells at a lower Force to keep Drain managable. It's not like low Force spells do nothing... in many cases Force 3-5 is already very good.
Besides you can get 2 extra dice for using a fetish with a spell (must decide when learning the spell), and you can get Positive Qualities to help against Drain, and I'm sure there's more.
And you can always use Edge on a few tough rolls, or when you really don't want to take damage in a situation. Did so just recently, when summoning a Force 4 spirit produced a rather unexpected 6 DV Drain... soaked it all up, thanks to Edge.

Bye
Thanee