Call For Backup has so many ways to give a ton of +1/+1 counters to your creatures, which makes practically every card in the deck a giant threat
Where Wyleth, Soul of Steel and Akiri, Line-Slinger used to reign supreme, Eivor, Battle-Ready actually gives you a win condition built right in
Wizards originally planned to release Spider-Man as a much smaller set with just 100 cards, but shifted that strategy in the middle of the development process
Sivriss also lets you run a background, so you can shape the deck to fit different playstyles like a classic aristocrats build with Revel in Riches and Pawn of Ulamog or an artifact recursion engine with Myr Retriever and Trading Post
Still, he offers some inherent protection by being able to counter spells, and abilities like this are specifically harder to nullify