I think it'd be more accurate to compare Seagal to them. In which case you'd identify that Seagal has had the most pidgeonholed career, playing essentially the same role over and over again. He kind fits into two different roles, the (ex)military/law enforcement guy, or the eco warrior, or both. And he always gives pretty much the same performance. Certainly that may not be such a bad thing. He's got his rep, and he sticks to it and delivers.
It's also not a great idea to compare him to guys like Snipes, or Stallone, who have shown in parts of their career, to be fine actors. Seagal has never shown that. Where Seagal will come out favourably in comparison is his "I'm the man" like aura he had when he fought (in his movies up to about Glimmar Man). Seagal in films like OFJ was a total badass, the coolest kicker of ass since Bruce. Most of the other action guys went in for that whole 80's homoerotic thing, with the shirts off and the oiled pecs.
As for scripts, Seagal's only real decent script was Under Siege. It may have been a tried and tested concept, but the script was brilliantly paced and with intelligence. Too many of his other films follow generic action patterns, and are all very much like for like. One's he wrote himself (ODG) are overwrought, unrealistic. However the script isn't the selling point of an action stars film. On a script basis he falls on a par with Jean Claude Van Damme, and Dolph Lundgren who similarly were doing films that followed generic action patterns started by Norris and Bronson. However in Seagal's advantage he had the one superhit, and real quality piece in Under Siege that JC, and DL (and Norris for that matter) didn't have.
Authenticity wise I don't believe that really any of the action guys ever really played authetentic, or believable characters. Occasionally someone like Bruce Willis would come and do an everyman action film. In regard to authentic actions Seagal probably wins the day. The way he holds a gun, the use of his martial arts on film (not like Van Damme building up to his helicopter kick and the end of all his early films) are far more realistic, due to his martial arts expertise and not trying to moviealise it, and also due to his rumoured CIA past, that may have helped.
