No major downside that I'd veto the MR over. Imho a set is better for two reasons:
- It ensures by design that there are no duplicate elements ever.
- It makes it immediately clear to any casual reader that there can be no duplicate elements.
Of course there's the whole "hashtables have O(1) complexity compared to O(n) for lists" thing as well, but on a small scale like this with only a handful of elements, I'm not concerned about that.
If you really want to keep the list, if only to avoid the work of switching over, I won't be throwing a tantrum over it.