No way i'd let mine go to one at that age
why not?
Firstly, I don't believe that any parent anywhere has a 'completely open and honest' relationship with their teenager (as much as so many on this thread like to think they do} 
. There will always be stuff your teenager hides, stuff they do that they won't tell you because they know you'll disapprove. Just think back to when you were a teenager and got up to- did you tell your mum everything? Parents rarely know everything that's going on.
For me, allowing a 14 year old to go to a mixed sleepover is a serious lack of parental boundaries. 14 year olds are often horny, hormone-fueled and thoughtless, even the really sensible ones. Chucking a group of teenaged boys and girls in their PJs into the same room overnight and leaving them to it (because even at a 'supervised' one - is the parent bedding down in a sleeping bag with them? Or staying up all night to ensure no one sneaks into the other room? I doubt it) is asking for trouble IMO.
IME, teenagers generally push 'the next step' when they're rebelling. A bit beyond what they're regularly allowed to do, a bit beyond their boundaries to test the waters. A 14 year old with extremely religious, straight laced parents with a strict 7pm bedtime probably isn't going to first rebel with an out of the blue all night drink and drugs fest. They might rebel and say fuck you and stay out till ten with a can of larger. The teen who's allowed to stay out until midnight though, or allowed to go to mixed sleepovers or allowed to have safe sex (all that 'i'd rather it was under my roof bollocks which I hate) and allowed to get tiddly with mum on a Saturday night - how do they rebel? That stuff's already acceptable, as endorsed by the parents. As part of this wonderful 'open and honest and adult' relationship that they have with their 14 year old 
. But that 14 year old will find something the parents disapprove of which is likely to be a whole lot worse if the parents already allow so much.
Me and my bff as an example. My parents were fairly strict when I was 14. Minimal alcohol allowed at home (a sip or two). NO alcohol when I was out. NO cigarettes. The thought of drugs would have had my mother on the bloody roof, she was vehemently (scarily so) against them. My bff's mum was the 'open and honest' type. She was allowed to drink at home (within reason) and allowed to drink out of home at parties (again, within reason because my bffs mum 'knew' she was sensible). Her mum also turned a blind eye to her smoking.
We rebelled together at 14. I hid getting pissed and the odd cigarette from my mum when we were out. My bff hid coke and spliffs from hers. We both took 'the next step' as according to our boundaries.