I've been running a wizard in Shilsen's campaign with full access to all PHB spells and 2 SC spells per level.
In theory, it is very powerful.
However, it really hasn't changed much of anything from a practical point of view. 99% of the time I'm using the spells I'd have taken/acquired anyway.
In reality, you don't know what you're going to be encountering. The number of spells you can actually use daily is very limited and you have a lot of bases to cover (defense/offense/information/crowd controll/buff/etc...). You have to try to be prepared for a range of possible foes, attacks and counters. My 16th lvl char with a maxed int +6 bonus to int, essentially only gets at most 7 spells per spell level and that number falls off very sharply for the the highest level and thus most useful spells.
Poor spell selection can render you either ineffective or impotent.
IF you alway knew what you were going to be facing.
IF you always had the opportunity to prepare a tailored list.
IF you had a lot more spells or the opportunity to choose on the fly what you could cast.
Then yes it might be unbalancing.
But unless that's the case, I wouldn't worry about it.