in a PUG i've been having great success with assasinate and sneak attacks. i'm a mechanic with a repeater so the melee assassinate ability takes a bit of getting used to with all the weapon swapping. but it's easy enough to get the sneak attacks in, just takes a bit of practice to land the assassinations.
one big help was finding out the poison stance negates sneak attacks (thus assassination attempts) on unaggroed mobs. the poison damage applies first, which gets the mobs aggro, which blocks your ability to sneak attack. so try turning that off and see if it works better. also as it's a fort save you can expect it to work better on casters and be more likely to fail on melee mobs.
when i soloed stealthy to see if there were issues (like kobolds seeing you through doors) i managed to avoid alerting the prophets. i did get a bit of unwanted aggro due to kobolds screaming as they died, which woke a few kobolds in the local area up, but with the new mechanics i managed to contain the problem and avoid the situation escalating as it has for me so many times under the old system. once the quest was complete i then went back trying to assassinate as many kobolds as i could and for the large part did it without raising any noise.
the system really can work, and if you mess up running out of sight and re-hiding does let you go back to sneaking when the mobs calm down. it's a massive improvement to the submarine mobs of old and i love it!
also the spot over time buff they get for looking right at you was always there. there is a very old youtube vid of a sneaky mage demonstrating that effect in the orchard. the only difference is now we can see they are racking up the spot bonuses. so i use that to quickly cross a room when the guards wont turn their backs and to know when i'm behind cover.