Hard to sneak in Burial chambers as I think they can hear you open doors. Not sure on that but it seems to be my experience. Combat will also alert nearby mobs as well.

I think, IIRC, sneak skill also reduces the stam you use while sneaking, or the speed you move at. Maybe both.

I don't use sneak an awful lot anymore, but did with my first playthrough as a dagger user. ATM my sneak is maybe 10-15 tops in Mistlands. If I do sneak to backstab a mob I can usually manage it outside as long as I'm behind them and watch the eye meter and noise I make.