Some of the frustrations from Shogun 2 are present in FotS, namely that allies have an extremely high chance of betraying you, even if they're vassals you liberate from factions that they hate. It's much more rabid in FotS than Shogun 2 though, for a couple of reasons.
The first is that realm divide happens much earlier in FotS than TWS2. In TWS2 you could capture roughly 20-23 provinces before triggering realm divide, which is roughly a third of the map. If you allied with someone beforehand and keep them happy, you can effectively conquer half the map with little difficulty. In FotS, you're looking at more like a fifth of the map being on your side before realm divide. All factions with the same allegiance as you become permanently locked as allies, while all others become permanent enemies. The problem is that, on Legendary, practically all factions quickly convert to the allegiance that's not yours.
The easy way to fix this is to just flip allegiances and then promptly attack your former allies...but this just feels really lame and easy, not to mention ahistorical. The alternative is to then slog through 4/5ths of a game map filled with enemy armies (which is fine) and fleets bombarding every province and port you have nonstop (which SUCKS). Forums recommend either blockading and bombarding every enemy port constantly, which sounds like an excruciatingly long and painful process, or to build a chain of fleets blocking enemies access, which is insanely expensive, particularly when you only have a fifth of the provinces.
So one way to counteract this I've tried to do is recruit a whole bunch of ishin shishi agents and just inciting revolts everywhere. This method in TWS2 was so OP that you could effectively turn the entire map into rebel provinces before RD. The problem is that where you can easily make 50k a turn pre-RD in TWS2 and have missionaries that have a 70+% of inciting rebellions and usually costing about 800 per try, in FotS you're making more like 9000-15000 per turn pre-RD and agents have a ~50% chance of success even if speced specifically to incite revolts, and cost 1000 koku per incite attempt. This means that you are blowing through some 30-60% of your income just to incite revolts, half of which won't work per turn. Rebellions are also weaker, usually just featuring a bunch of levy infantry, while enemy armies are just everywhere with a bunch of uber units ready to wipe them out.
The last method I can think of is to expand extremely--slowly--being very careful to not discover any new clans until everything is heavily upgraded. If they haven't discovered you, they can't declare war on you, so that's something...the extremely guarded approach also means that by the time you moved out you would have a bunch of techs researched. The big problem is that this doesn't stop the problem of basically every clan converting to an enemy allegiance, plus the concern of the campaign running out of time before you can complete your objectives.
FotS is so frustrating partially because I think the actual combat is pretty good. If only they made it so that all the clans are bizarre backstabbers who do anything to kill you it'd be a great game. In my Satsuma campaign I allied with the clan owning Tsukushi. Then Tanegashima, my vassal, declares war on me. A turn later, my ally declares war on me. The following turn the Obama and Jozai, clans that are on the opposite side of the map and had only met me two turns ago, landed doom stacks in Bungo and started shelling my provinces. Is this supposed to be fun??
I get that CA was probably trying to make the late game of their total war games more exciting. Previous games' late-game turn into snoozefests as your invincible armies just overrun everything. But surely there's a balance that can be struck here.