I guarantee you that it is nearly always experienced as a racist statement by the person hearing it, which is the entire point of this thread.
Erm, you can say it to someone who is the exact same ethnicity as you, so that's ridiculous.
There's a thing called tact, judgement, you use it to understand which words to use in which context. Of course saying it to a person with a different ethnicity could be considered racist, especially if you don't know the person.
Choose your words based on the situation.
You might not consciously be thinking "oh, this person is 'strikingly unusual'", but as you point out, that's what the word you're using actually means. Not only is calling somebody unusual not a compliment, I don't consider myself unusual. So what you're really saying is "I think you're weird because of how different you are from me, but I like that."
How nuanced!
That's not what it means at all.
It means your look is unique, you stand out, some people would definitely take this as a compliment, others wouldn't. You use language combined with tact to judge these things.
I know a tansgendered guy who dresses incredibly flamboyantly, if I called him an exotic beauty he would appreciate it. I know this, because I know his personality.
It's about context, not all words are always suitable, but that doesn't mean they're inherently bad.
damn, girl, you so strikingly unusual
you so strange in effect
you so strange in appearance
EDIT: Let me re-phrase: Yes, it could be taken negatively, and putting it in those exact words doesn't sound great, but you could also write "I think you look incredibly unique", and there are people who would take that as a compliment.