In fact, let's really amp up the magic ante by adding Jack of All Trades (Mage) to the list of crazy strong and likely broken heroic abilities. Want to be good at your profession AND roast people with magic? Now you can after only a single session! Note that the Mage ability specifically states you become trained in a magic skill. Regardless of whether your base profession has it or not.
The anti-magic properties of iron and steel kind of negate this. Now your fighter needs to choose between using their sword or casting spells, the Knight between wearing armor, or casting spells. And so on. The rules intent seem similar to previous editions in which you couldn't even carry iron (or steel) items such as a lantern in your inventory or reside within an iron cage and also cast spells (a quiver of iron arrow heads are clutch, rules as intended I'd say a no, but probably yes to most GM fiat, as it's"small items").
I dare say this solve most of the problems invoked by picking up Mage, as it would seriously hamper the utility of your previous profession.