Maybe it's just because I place more value on having confidence that something will work, rather than having the possibility that it will work. I like to reduce down the number of contributing factors. For example, sure you can spread all your units out, but it's actually quite difficult/impossible to do within the deployment zone (most Eldar can't garrison) whilst also staying out of sight. Call it laziness

The whole artillery issue is less of a problem when using an air assault/planetfall list though, although the target then becomes your flak. You can always hit at least 2 units with a barrage no matter how you deploy them, which for fire prisms is often enough

I wouldn't say the VS is the best artillery in the game necessarily, I haven't played all of them. I do think it's a better counter-battery option than a spacecraft, which on the balance of probabilities is probably going to miss the artillery.
Ulthwe might well be better than Biel Tan, the SR5 and 1+ Farsight black guardian combo is quite good... Total beginners might struggle with guardians but after a couple of games you figure out how important it is to position them correctly in an engagement. They you have an army that finds it actually pretty easy to execute the rolling assault. Epic UK already nerfed that 1+ thankfully. It's also true that it can be frustrating for Biel Tan not having access to aspect troupes, but some of the aspects really need those numbers to get their overwhelming odds in winning engagements (see laziness above). In any case I like to have at least 2 aspect formations, which for Ulthwe might mean having one too many assault-themed formations once the necessary hosts are included. And aspects are just more "in theme" for Biel Tan. So pretty much just personal preference for me.
Then there's Iyanden, which I resist playing with as I don't think it's fair on my opponent. We have a joke that using Iyanden is the same as cheating

If your gaming group generally uses Ulthwe or Iyanden, maybe that's contributing to you thinking NetEA Eldar are generally too strong?
How is playing Iyanden or Ulthwe cheating sorry for being off topic.
As far as Void spinners go I have always thought and will all ways think it's over priced and a very easy target to take down.