What are the specific M1S problems?

[mooster1223]

I'm ready to put together a 300/221 or buy a bonified 300 whisper. I've already found out that J&T won't thread the barrel 5/8 24, M1S has a questionable reputation and SSK uppers are PRICEY!

I really don't want to "waste" money on the M1S upper if it's going to be nothing but trouble. Are they truly unreliable? There is nothing worse than getting a new toy only to find out that it's going to take lots of tinkering to get it to run. I want reliability with subsonics.

Are SSK uppers totally reliable and worth the extra money?

Any help would be great.

Jim