For me the show makes it quite clear Buffy and slayers in general are stronger then vampires. Like if she sat down and had to arm wrestle, Buffy would beat them every time. why make a slayer to kill vampires and make them weaker then vampires?
Angel said she was stronger, and she could lift Olafs hammer when Spike couldn't.
But every so often a vampire can get the upper hand in a battle. It's not by strength they win, just the luck of the fight.
I'm not sure how much stock to put into what Luke said, since he said it in the heat of battle. But then again according to him, the last time someone walked away from battle with him alive was 1843, though even then it was a guy, so presumably mortal not even a slayer, so if he thinks he's stronger then Buffy then a mortal, even if he caught Luke sleeping, shouldn't have been a problem at all.
We dont know how old Luke is, if he's older then Darla, it'd make sense he'd be stronger then a regular vampire and he'd have been one of the oldest vampires on the show, but I dont think he's as strong as he thinks he is.
Outside of Vampires though, there are a few demons that have been stronger then the Slayer, so when you think of Adam as a composite of multiple demons and part robot it makes sense that he'd be stronger then her