What does "better" mean to you? I've found that SX necks were on the thick and chunky side compared to Fender spec necks, which is not a plus to me. Mighty Mite necks will fit into a Fender style body a little better than an SX neck will because of the heel shape difference. The SX has a more durable finish (bulletproof thick poly shell). Oh yeah, don't get us started on another SX headstock discussion.
I think Mighty Mite necks are a step up from SX necks, and if you're building a "parts" bass that conforms around Fender specifications you should stick with Fender-spec parts. The SX stuff is nice for the price but the dimensions are usually off enough that you shouldn't use an SX neck with anything other than other SX stuff.
I have several SX basses, and the necks have held out well in all respects for several years. Good wood, good construction, they play well and are easy to set up and work on. I've done fret levels on many and they're comparable with almost anything else I've worked on. Zero complaints. I have a guitar with a birdseye MM neck and it's no better or worse than any of the SX necks. It's not impossible that they're made in the same factory in China. If you want a neck to avoid for inferior quality, I'd say to stay away from anything that says Squier Affinity.
They're good on SX basses. I think it depends on the SX neck. I've only ever had good luck with them myself, but I've never tried to match one up with a non SX body.
