-- They very obviously crammed two seasons' worth of plot into one season. There is no time to breathe, and lots of things now have to happen in an ultra-contrived way in order to work out. Also, skipping a year is rarely a good idea, and in this case, having two seasons, taking place over the course of that year plus the duration of this season, would have solved all the major issues.
-- A big issue is that the absolute high point of this season is the contrast between normal good guy Smith in the one TL, and monstrous nazi Smith in the other. Sewell being a phenomenal actor, he does everything he can with it, but there's not enough time to develop it properly. So much more could've been done with this.
-- The skip also robs Juliana of a more gradual journey towards... Enlightenment? We don't get to see it, and if her whole meditative journey had gotten much more space, it would've been far more meaningful.
-- Also, Tagomi just gets offed. Big disservice to the character. A fourth season ending with that, and then his first vision-appearance to Juliana-in-exile, would have been perfect. That would've set the stage for a well-contructed season five.
-- Black communists? Whence? I like the idea, but they weren't ever mentioned before. Again, just an extra season showing them coalescing first would've solved this. (Although frankly I'd have liked to see more of them earlier-- unlike most of the 'white' resistance, these guys don't do lynchings or casual murder of suspected traitors.)
-- Nevertheless, considering any even vaguely realistic take on the Pacific States' demographics, the BCR can only be a very tiny splinter group. Bringing them into the story is cool, but there's no way they end up in charge of the country. Maybe they'd get a real place at the table as part of a broader commie resistance, but I doubt even that. (Also, there are zero signs that any other part of the resistance is communist in any way at all. On the contrary: they all seem to idealise pre-war "Americanism", with lots of notions about individual freedom. It seems likely to me that a country built by a victorious resistance would be vaguely libertarian -- meaning right-libertarian -- rather than anything resembling communist.)
-- The Japanese withdrawal from the Pacific States is ludicrously hasty. Again, an additional season spent extensively setting up how bad things really are in China would've made this much more credible. (Also, they should've explained more: for instance, making it explicit that the Pacific States are the least important Japanese colonial holding, and therefore first to be cut loose if push comes to shove.)
-- Of course, in a realistic scenario, Japanese withdrawal means the Reich rolls in, so instead of withdrawal, you'd see self-government as a Japanese client/vassal. Police functions given back to the Americans etc. but still very much a country in the Japanese sphere, with Japanese nukes stationed there and pointed at the Reich. Also, this self-government would be handed over to white Americans, not to the BCR.
-- In fact, since skin colour can't be hidden, we know that the Japanese of this TL would've just exterminated all blacks as a response to the BCR bombings. It would've been very easy. The series creators obviously just didn't want to go there, but... let's be real. That would be the outcome here.
-- On the other hand, they clearly did include the notion that the nazis are far, far more technologically advanced than OTL democratic America. This carries the "nazism is efficient" under-tone, which is... not accurate in the slightest. It would've been interesting to see the nazis fumbling around a lot more when it came to inter-world travel.
-- The whole idea of "Americanism" among American nazis is very interesting, but again horribly rushed. Showing more of this (again, across two seasons) would've been much better as a set-up.
-- Smith's second-in-command calling off the attack is just absurd. He was the most enthousiastic proponent of re-uniting America! Smith gave him the order to do it! Does he even know Smith is dead at this point? Makes no sense. As mentioned, in a realistic scenario, the Japanese would've already killed all blacks in the Pacific States by that point, and would be pulling back a bit, instead of pulling out. There would be no war, and men like the "Americanist" officers would be living in hope of re-uniting America when the Japanese weaken further (which seems likely). Smith was already rather Americanist himself, and his death would put the Americanist faction in charge.
-- The realistic ending of this series would've been the gradual pull-back of both Germany and Japan, with the prospect off an ethnically cleansed, collaborator-ruled America re-uniting in a few years or so. Probably peacefully. I get why they didn't do that, but that is the expected outcome here. In fact... given what we've seen, I expect the Americanists to take over the American Reich on short notice (thus taking a lot of wind of the resistance's sails), and the BCR to be removed from power in the Pacific States. Those West Coast death camps may well end up getting built after all.
-- A more hopeful alternative would be to end with the resistance taking over the Pacific States as we hear reports of a huge Japanese defeat in China, with all the remaining Japanese forces hastily pulling out of the Pacific States... and the resistance gaining control of their nukes. Still absolutely no chance of the BCR being in charge, but the West Coast would then be a liberated, multi-ethnic, set-to-be-democratic country. Presumably with a lot (if not all) of the neutral zone joining this country. You could end with Juliana & co making it to safety in the west, after having assassinated Smith and blown up the whole nazi portal base. End the series with them receiving news broadcasts of an Americanist coup in the East, with it being stated that the SS is being disbanded. This would be realistic, but offer at least some real hope.
-- The actual ending, with a lot of people suddenly coming through the gate, is just total nonsense.
