I have a Kobo, not a Kindle. But I have seen the weird space issues on my Kobo (using kepubs). On the Kobo at least it has to do with the hyphenation dictionary and where it will or won't break words.
On Kindle (until recently) there wasn't hyphenation. That's probably where the weird spacing complaints came from. As Cin said, that's changed.
I guess what I'm saying is I wouldn't let spacing concerns steer you away from a Kindle.
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