When luck is good, then PPLNS system is great to be on.   When luck is bad, then PPS is better.

Luck can change at a moments notice.    

I personally am not worried about luck.  My concern is what if something is broken and we can't find a block?   Computers and systems break all the time.

Another thing always bothered me as well.   Every time someone else finds a block, we have to restart our search.   Perhaps all the restarting is causing an issue?  

Again, forgive my ignorance.