Fanned frets won't help much with intonation as such, though they do allow you a slightly longer scale for those low strings which means you need less compensation (in relative terms), but it doesn't mean that the string will intonate any better.
I've only limited experience with them (I made myself an acoustic eight string some years ago, with two additional frets on the low end of the three bass strings, kind of like on the Kubiki X-Factor basses Stu Hamm used to play, but on three strings instead of just one). The big thing about them is tonal and ergonomic. The different scale lengths evens out the tension on the strings, so they are a little more even from string to string (at least, if you string it with the right strings). I had a fairly long talk about this with Ralph (Novax, who invented the whole idea) when I got my fingerboard from him (I normally make my own, but I couldn't think of any way to make one of those things right).
The main thing though is ergonomics. They are shockingly easy to get used to (or at least, I thought so), and really do make a difference. Personally, I would not want to play an eight string without it. It makes the hand fall in a very natural alignment with the frets at all areas of the fingerboard. Down low, when you hand wants to angled away from your body, the frets are just that. When you get closer to the center of the fingerboard and everything is lined up straighter, so are the frets. When you get way up high by the body, you hand naturally wants to angle towards your body a bit more, and the fingerboard lets you do so. However, the ergonomics of it kind put a limit on the tension and tonal issues, because any more of a difference between bass and treble scales than about 1" is very uncomfortable to play. An inch is a lot, but if you are going for a low f#, it might not be enough.
One other issue to think about. All those ergonomic issues are for guys who wear their guitars pretty high. If you wear your guitar on a really long strap (ala Adrian Belew), none of that geometry is going to work out the same way. Just something to keep in mind.
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