It’s a brilliant illusion that works… until you hit aliasing, memory models, or concurrency, and suddenly the beautiful DAG collapses into a pile of phi nodes and load/store hell.
Mutation is the result of sloppy thinking about the role of time in computation. Sloppy thinking is a hindrance to efficient and tractable code transformations.
When you "mutate" a value, you're implicitly indexing it on a time offset - the variable had one value at time t_0 and another value at time t_1. SSA simply uses naming to make this explicit. (As do CPS and ANF, which is where that "equivalence" comes from.)
If you don't use SSA, CPS, or ANF for this purpose, you need to do something else to make the time dimension explicit, or you're going to be dealing with some very hairy problems.
"Evil" in this case is shorthand for saying that mutable variables are an unsuitable model for the purpose. That's not a subjective decision - try to achieve similar results without dealing with the time/mutation issue and you'll find out why.