One major issue is that providing enough fusion power with ARC reactors to power the world economy would require 100x more beryllium than the current estimated global resource (not reserve). Only a bit over 200 tonnes of Be is mined every year right now. It's not a common element.

Abdou's team (the fusion engineering guy at UCLA) rejected molten salt blankets for this reason, among others, after trying really hard to get them to work in studies.

One big problem with fusion is the low power density. I harp on that a lot, but it's been known to be a very serious problem for decades. ARC's power density is 40x worse than a PWR's reactor vessel. It's difficult to see how fusion can beat fission given this. I suspect the optimistic numbers for fusion come from using a way too cheery cost estimation methodology, something that would predict fission is far cheaper than it actually turned out to be.