Walken's casting is a mystery...in the movie, his acting job is actually good, he portrays the disgust and cynicism of such a character well, but his physical "presence" kills it. He is, to be blunt, far too skinny, pale, and sickly-looking to be believable as a hardened, professional soldier-for-hire. He looks as if he could collapse and die at any time, and it is difficult to buy him in this role, especially after he survives a vicious beating at one point that would have killed a man who looks so poorly.
Nonetheless, if one overlooks this glaring imperfection, The Dogs Of War stands up quite well in the Merc Movie genre. Even better in this underrated action movie genre are the aforementioned Dark Of The Sun, starring Rod Taylor as a Congo merc in the '60s, Ronin, starring Robert De Niro, Jean Reno, and Sean Bean as 90's, post-cold-war mercs in Europe, and The Wild Geese, starring Richard Burton and Roger Moore in an adaptation of the original "Wild Goose", Mike Hoare's book of his own Congo exploits, only the movie is set in the '70s.
Taken together, they are fairly accurate and interesting accounts of their various historical periods. I recommend them highly.