Tripeace is one of my favorite mangas, i read it back in 2012 and every few years i go back and re-read the series (I'm on my 6th re-read) so i pretty confident to say that Tripeace doesn't hold up that well like most stuff from the early 10s and late 00s.
Art is just amazing, don't even try to argue. For something made in 2008 the art is amazing.
Story has a very great premise but it bite off more than it could chew so most of the talk about war is very simplified for what it was trying to say which came off often as childish and naive.
Characterizations for nana and shiro is pretty decent, nothing revolutionary but it serves the purpose. Nana has amnesia so he's a blank canvas with the goal to stop all wars and for essentially a young boy who just learned about what war is, he did his best (the whole idealistic tendency and childish tone of the story towards war fits nicely but war isn't really something you can use the word childish with). Shiro's development is nice and the answer she came up with for her third law is quite surprising (or if you pay attention enough and is smart maybe not). The antagonists are often contradictory in their words and actions which reflects the muddy line of good and evil (i hope that was intentional). And the rests are not explored enough to say anything about.
Pacing has issues, let's just say that. Especially the last arc.
First time reading the ending i was really disappointed but after re-reading and look at the story as a whole, it is still disappointing but i appreciate it a lot more. Nana growth is comparable to a kid growing up (tripeace is a shounen), he has an idealistic view of the world and when it was broken, he succumb to negativity and becomes disillusioned with the world and himself. But when he and shiro fought, she pulled him back from the despair by reminding him of his dream. At this point, both sides are fighting each other like kids at a playground because they deep down are still idealistic children no matter how strong they are.
The ending to me is about staying true to yourself. Nana dream is to eradicate all wars and even though it's an impossible goal, he still strive to do it because it's what he set out to accomplish to succeed or die trying and i think that is beautiful. Very heavy-handed in terms of execution and cliche in meaning but it fits the story and stay true to itself till the very end.
I'm still very sad we didn't get to see the fights though... And the set-up which dragged on pretty long without going anywhere... And the humor which DID NOT aged well like soul eater...
Tldr:
Pros: Cool art, easily binge-able, compelling story
Cons: Humor, rushed last arc, all over the place set-ups which lead to nowhere resulting in a sense of pointlessness.
Overall: 7-8/10 (to me it will always be 10/10)
Read it if you want to kill time or to come up with ideas for your own comic because it is filled with cool designs and concepts.