The issue is that such a Flaw would not be balanced unless the penalty were much harsher and/or the scope were much broader than a corresponding Focus. That's because a magus with a "positive" Focus will ... focus a lot of his magic within it, while a magus with a "negative" focus will try to have all his magic outside of it.
For example, Incompatible Arts is more than twice as broad as Minor Focus, and it completely prevents a magus from producing magic within its scope rather than "just" penalize him ... and still, Minor Focus is widely regarded as a strong Minor Virtue, while Incompatible Arts is often seen as relatively benign Minor Flaw.
"You guys won't believe just how yoked my familiar is."
But yes the flaw is an absolute mess, although I don't think it's an insurmountable limit to play.
In the first place, mMF(certamen) is such a terrible focus that I fully refuse to play Tremere purely on that basis. But if your mage really was so gimped as to be myopically focused on Certamen, he'd be a huge-grandstanding-clout-chaser and therefore a consummate tremere. Which is probably what you're going for when you play that house anyways.
Now MMF(Familiar) has some really interesting opportunities for play because it involves virtually ever Tech:Form combo. And the only limitation you experience when Myopic about it is that all your focus has to go into making your familiar as abominably yoked as possible. So really all you need to do is either get an Attack Elephant or a dragon or a really powerful spirit familiar/daimon and take things from there. Which, all things considered, isn't really such a bad shake.
Not to say that the flaw isn't totally busted because what do you do when the focus you choose is astrological?