That show was deeply flawed in their simulations, as they only allowed a given combatant to use their iconic weapons rather than their full arsenal, and each weapon or tactic could only be employed once per simulation; as evidenced in "Nazis vs Vietcong" where the Nazis used a flamethrower once and its never addressed again, or in "Mexican Cartels vs Somali Pirates" where the Cartels fire their AK-47s ONCE early on and it's never seen again for the rest of the simulation.
Also, some of their matchups were just screwed up, like "Ninjas vs Vikings" in which the Ninja engages the Viking in open combat without using any stealth tactics that were, oh I don't know... THE ONLY THING NINJAS WERE EVEN CREATED TO DO.
By comparison, Death Battle is much fairer, as it allows fairly full arsenals (restricted only by the principle of "we'd be here all week if they used the actual full arsenal"), repeat tactics (watch how Kirby spams his eating people and transformations) and when they DO cycle through stuff, it's because the previous methods weren't working instead of an arbitrary "one-use limit". A great example is in "Gundam Epyon vs Tigerzord where Zechs hangs back and analyzes the White Ranger's tactics with the Zero System, and only goes all-out on his attack when the White Ranger ran out of new tricks and was forced to begin repeating old tactics, which meant that the Zero System could now account for and anticipate every capability the Tigerzord had (which is perfectly in-character for Zechs in Gundam Wing).
Death Battle has some major issues, but it's nowhere near as fundamentally screwed as Deadliest Warrior.