Seems like you already got the replies you needed, but I'll go ahead and add my two cents anyway.
I'm riding relatively cheap boots (Vans Encore BOA) with a single BOA system this year. I've got ~12 days riding them so far this season and love them. I did not pick them because they were cheap, but rather because they fit me best of the boots I was able to try on.
I see a lot of hate for the single BOA boots but have found mine to be plenty comfortable, to the point where I put them on before we drive up the mountain and don't even bother loosening them when I eat lunch. The key was figuring out that if I flex the boot a few times while tightening it up it distributes the laces across the whole boot more evenly and lets me tighten things down properly in the first go. Sometimes I'll need to tighten them a bit more halfway through the day because my feet have settled into the boots more or I didn't flex them enough when putting them on for the day but the BOA system itself has never loosened up on it's own.
All that said, my husband had the BOA system on his K2 Maysis boots fail 5 days into the season. The boots are double BOA, with one controlling the boot tightness and the other the liner. The one controlling the boot failed to properly release when he went to take the boots off one day (it would release about one inch of cable but no more) and the only way he was able to get the boot off was by cutting the cable. We were not able to get them repaired on the mountain and he had to just rent boots the following day.
You can pick up a full set of replacement parts for $12 + shipping from BOA if you want extras to have on hand. If your BOA system actually breaks they will sent you replacements for free. There was someone on the mountain who would have been able to fix the boot for us if we'd had the parts, but the store there did not carry replacement BOA parts. He did mention that if it had been a cable that broke he would probably have been able to tie it back together as a short term solution to keep the boots running until we could get a new cable.