Cards like Budew completely stop the deck from functioning, and it has horrible matchups into decks like Raging Bolt ex and Gardevoir, both of which are dominant meta threats currently
Ideally, you should have something that covers one-shot artifact hate (Relic of Progenitus, Tormods Crypt, etc.), the interaction thats hidden in the opponents hand (Endurance, countermagic, etc.), other problematic permanents (Dauthi Voidwalker, Sanctifier en -Vec), and sometimes enchantment with static abilities (Leyline of the Void or Rest in Peace)
The converted mana cost of the spell remains unchanged, no matter what the total cost to cast it was and no matter whether an alternative cost was paid
Today, Wizards of the Coast revealed lots of new information about MTG Foundations, and the most exciting thing revealed may end up exciting Commander players more than anyone