US-Japan BR III stregnth comparison.
Rifles - Allied superiority, in both semi or bolt-action, more so in bolt-action, semis are a bit closer in power level.
SMGs - Roughly equal, both have ups and downs, no side has a obvious superiority.
MGs - Equal, both have essentially what the other side has (BAR/KE7, Bren/Type 96/99).
Manheld AT - Large US superiority. Japan lacks BR III AT alternative, the two BR II alternatives are very hard to use effectively against US BR III tanks. US has the M9, which is amazing.
Tanks - Large US superiority, The Sherman (any BR III variant) is a beast in the pacific, 105 is a very effective HE slinger, but so is the regular 75. Uptiered Japan has little to respond with, BR III tanks are still inferior, US tanks can can reliably pen any Japanese tank from the front whist the Japanese have to learn to aim in response.
Air - Equal, with caveats. They have their own stregnths that they play well to, but it’s obvious that when the US controls the sky Japan suffers more than if the situation was in reverse. At the same time, Japan has very good fighters that any skilled pilot can use to dominate the skies with (even BR II Zero is a beast). In order to reliably push out Japanese air superiority against a good pilot the US needs to rely on AA, ambush tactics, or bringing up Spitfires (event one included).
Other factors to consider:
Enginer structures - AA is equal, Japanese AT feels weaker (citation needed), US HMG is stronger (greater depression, more satisfying RoF), MMG is roughly equal (I prefer the US due to slightly higher RoF without being wild like the Germans, but both have very good depression stats which is the most important part I think to Engineer buildables).
Matchmaking - Japan BR III can’t be uptiered. Of course, only about five people play BR IV anyway, but it is a consideration anyway.
Maps - US Queues up against many more maps, meaning a dedicated Japanese player can learn the Pacific maps to a degree that the US one would have to spend longer on.
Players - US players lack willpower, give up too easily, prefers building AA guns over rally points, and plays snipers too often. Japan wins most of their games (personal experience) from US morale just collapsing too easily. Long story short, those who play as the US tend to be casuals, just playing one of the two main factions in the game (Germany and the US), whilst those who play as Japan really wants to play as Japan.
In summery, US has a lot of clear advantages, Japan has fewer, but they are closer in power level than I give them credit fore. US dominates in tanks, this is obvious, the rest is up for debate.
What makes US/Japan as balanced as it is though, is because the average US player doesn’t comprehend how to play the game, whilst the average Japanese player has fully accepted the Banzai mentality and therefor plays more aggressively.