Conversely though, it's also a big weakness for Yang. Part of what continues to make LOGH so special even over 40 years (!!) later is how it pulls no punches in its observation of humanity. Yang sticking so strictly to his ideals and refusing the path of melding with politics is on one side extremely admirable, but at the same time Clausewitz' old aphorism of "war is the continuation of policy with other means" does still have a role. No military commander, no matter how genius, no matter the morale of his men, can always win if the home front collapses behind him. Yang is unpredictable and challenging to others, but at the same time he has blind spots and is unable to react to some of the moves Reinhard makes. And perhaps most important, Reinhard unites the entirety of his side behind him, both militarily and politically. Yang doesn't, and it's not 100% pure either, he's a human too and he doesn't want the responsibility and helping of misery that comes with putting yourself in such a position.
In the end, that did mean that Reinhard won, and importantly this also goes back to the quote saying it's a "pointless" war: he also made the empire a much better place, at least for a little while, then it was before he began. As the story points to with that brief shot of education reformers, a spell of hope and positive reform can bear fruit for a long time afterwards even if leadership gets worse again. Which is all Yang himself had hoped for for the Alliance. "The ends justify the means" rightly gets a lot of skepticism, but that doesn't mean that no ends ever justify any means either. Some goals and values really are important, and worth fighting for. And if your goals and values are to stop all the fighting? Well you need a lot of power for that too, both military/economic power and political power.