I think this is true within reason. A good example that relates to this is the love potion example on YS281. In this example, a specific aspect is chosen at the creation of the potion, though it is the result of some negotiation between the player and GM. I think this sums to 'the wizard gets to pick it within some reasonable limit'.
That's more or less how I feel - if the effect sounds unreasonable for a mild consequence then make it a moderate one. Since every rite is more or less a custom one the wizard designing the spell is also designing the consequence.
YS282 has the rules for this. For the spell to work, you need to beat the target's defense roll with enough shifts to bypass their stress track and inflict the consequence. To guarantee success, you would need their defense skill + 4 (max defense die roll) + stress track + 4 (for a moderate consequence). If you use a less powerful spell, then they'd make a defense roll, and you might find that you only inflicted a minor consequence, or even stress only.
To me a spell meant to inflict a consequence is an all or nothing one. It either works as intended or it doesn't work at all.
For example, cursing a runner before a race to have the "Sore Leg" moderate consequence. If the defense roll is ---- then there are 8 unused steps in the spell. The caster doesn't get to add those extra steps to make it 12 steps of consequence and hit him with "Broken Leg" (severe) or "Lame for life" (extreme). If the runner happens to be behind a threshold or it's drizzling rain to the point where there's a -1 or -2 from the running water and the spell comes up two steps short, then the caster can't change his mind mid spell and hit the runner with a mild consequence.
And dropping the "maximum dice roll" steps can sometimes save a lot of prep and casting time. If you make the spell so it works when the roll is +1 or less then it will work 81.5% of the time. If you aim for +0 it works 61.7% of the time - and that's two less aspects you need to prep the spell. Of course then you risk it not working, but if you need to rush then you need to rush and hope you get lucky