That's the way I read it. That's the nature of the Major Magical Focus. It serves to define a magus. If my focus is gold, I know that all my items (largely) will be constructed from gold. That's quite limiting in itself as I'll probably not be the best swordsmith you're going to find.
A focus in "metal" is broader (still within the legal range) and probably alleviates that problem to a degree, but my enchanted brooms of sweeping aren't going to be as strong as my swords of swishing. And I probably wouldn't enchant an iron broom because, well, because that's just a bit rubbish.
Agreed. But, the question is, way more useful than what? In my current saga we have a Bonisagus who has an effective magic theory of 9 (I think). My Verditius is up there too. Now, out in the field there's very little the Bonisagus can't do with Ignem. That's way more powerful than my magus. But, inventing devices? That's my speciality. Especially metal ones.
Granted, my focus also applies to casting spells but that tends to be a little less important to me than the lab total.
It's usually the case that a player will pick a direction for their magus and explore that direction as far as possible. In the case of the Bonisagus, he already has frightening levels of Creo Ignem. For me, I get to enchant powerful effects more quickly. Neither of those (yet) have overpowered one character or the other and currently neither has proven to be more useful (I'd put money on it being the focus...) just different.