I think Beldin is making some very good points but neglects 3 downsides:
The first is the high spellbookcost for most mages.
Secondly, unlike all the other cards (Bull, ...) Sunfire amulet has to be played during the first 1-3 rounds or maybe at the latest turn 4/5. If played later, you probably won't get much use out of it. Of course there are exceptions to the rule, but the vast majority of the games tend to take 8-13 rounds.
This means that you have to play it during your opening where every spell you cast is mostly key for your gameplan. Setting aside 6 mana and a quick cast so early in the game is a big investment since that could also be used to cast a bigger buddy, an armor, a battle forge or anything else to help your main assault.
Thirdly, it can be countered by finite life as well which is mostly not the case for the other examples since they work immediately. (assuming you did not have a FD poisoned blood on your mage)