I think there might also be a UV degradation factor with Spectra line, but I don't actually know how long it takes to break down.
It's strong, for sure, and some people use it (in larger sizes) for standing rigging on sailboats,
but they also talk about a 5-8 year life span for that use, vs. double that for SS wire.
I guess on a bike that wasn't ridden all that much, it'd be OK, but I know I'll never find out, at that price.

Also, I wonder how long it would hold tension -
I've read some articles, that say that rigging made with the stuff has to be periodically re-tensioned, due to "creep"......

Also, I think it's ugly (for a spoke material) to start with, and I know for a fact (having seen and handled some), that it gets kind of fuzzy-looking after a while.