Maintenance for the week of February 22:
• [COMPLETE] Xbox One: NA and EU megaservers for maintenance– February 24, 6:00AM EST (11:00 UTC) - 10:00AM EST (15:00 UTC)
• [COMPLETE] PlayStation®4: NA and EU megaservers for maintenance – February 24, 6:00AM EST (11:00 UTC) - 10:00AM EST (15:00 UTC)
Both attack and skills, and you can use it from sneak,
However some skills benefit from it more, some less. For example dualwield flurry is a series of fast weak strikes followed by a more powerful strike. The ambush bonus is only applied to the first weak strike of the series(basically wasting it).
So if you want to get the best effect from ambush, follow it by a single powerful strike, like 2h uppercut, veiled strike, or a single-hit ultimate (soul shred, death stroke)