This conversation was spawned off from
this thread about Transformers Victory and Zone.
Because he's ruthless against his foes and takes no prisoners, which is unlike any other Autobot leader before him and many Japanese fans argue that it's against the spirit of what an Autobot leader should stand for. Using Convoy as a benchmark, they argue that an Autobot commander should always exercise mercy and compassion.
In Optimus Prime's G1 Universe profile, it specifically states that his greatest weakness is his compassion. But it also states that without that compassion, he simply wouldn't be Optimus Prime - i.e.: that it's his compassion that helps define his character. Grimlock was even less merciful when he was first leader - to the point where he became a tyrant and was merciless against fellow Autobots... but the difference is that many Autobots under Grimlock's command were unhappy with his rule and it ultimately led to acts of desertion, mutiny and a limited Autobot civil war. Dai Atlas on the other hand is a widely celebrated leader who is loved by his Autobots and allies - to the point where Star Sabre allowed him to assume Autobot command while he went off the search for Convoy's body (so that he could be rebuilt as Star Convoy).
From a military POV I think Dai Atlas' leadership style makes more sense. As cruel as it may seem, that is sadly the nature of war. As Optimus Prime's G1 Universe profile states, his compassion is his greatest weakness and it's a weakness which Dai Atlas doesn't have because he simply holds
no compassion toward Decepticons. Unfortunately in war you just
have to incapacitate your opponent's ability to continue fighting as quickly as possible.
There are more and less compassionate ways of doing this. For example, the Cybertronian martial art of Diffusion is preferred by Autobots because it allows them to incapacitate an opponent without having to hurt them (it's primarily about submission holds and locks). Likewise with Jet-Judo.
Diffusion

Dai Atlas' fighting mentality is more akin to what Decepticons would prefer and uncharacteristic of an Autobot, especially an Autobot supreme commander.