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Mutable structs are not evil.
They are absolutely necessary in high performance circumstances. For example when cache lines and or garbage collection become a bottleneck.
I would not call the use of a immutable struct in these perfectly valid use-cases "evil".
I can see the point that C#'s syntax does not help to distinguish the access of a member of a value type or of a reference type, so I am all for preferring immutable structs, that enforce immutability, over mutable structs.
However, instead of simply labelling immutable structs as "evil", I would advise to embrace the language and advocate more helpful and constructive rule of thumbs.
For example: "structs are value types, which are copied by default. you need a reference if you don't want to copy them" or "try to work with readonly structs first".
It doesn’t have anything to do with structs (and not with C#, either) but in Java you might get problems with mutable objects when they are e.g. keys in a hash map. If you change them after adding them to a map and it changes its hash code, evil things might happen.