There's a lot of stuff going on with Vraska, Betrayal's String that makes it worth the steeper price point, such as the aforementioned Poison Counters, the Proliferate effect, and even the ability to neutralize threats by turning them into treasure artifacts with its most costly attack, this card can end the game with its Poison Counters alone
Similarly, card types like planeswalkers are necessarily found across all colors
It's not a highly efficient source of damage, and by playing it, you're not playing the burn spell that may finish off the opponent
Sometimes, but most people are happier foiling selectively
The equip ability on its own, without outside help from cards like Leonin Shikari, cannot be activated at instant speed, so if you play an Artificers Hex on an equipment thats already attached to a creature, they will not be able to remove that equipment before they are in their main phase, guaranteeing you a dead creature