As far as advanced units go, attack transports are one of the most powerful pieces you can buy. I wouldn’t go out of my way to develop them for any nation other than the US, Japan, and to a lesser extent the UK. For only 2 IPP more than a regular transport you get a unit that can drop any of your units off on the first round of combat and you don’t suffer double casualties. That means you can purchase a regular infantry instead of a marine for 1 IPP cheaper as an equal unit thereby lowering the cost of your attack to compensate for the higher cost boat. More importantly, you can attack with artillery, tanks, mechs, or any other ground unit on the first round like in A&A. Huge advantage.
The US would greatly benefit from a heavy Carrier too because it can carry 3 planes across the Pacific for 18 IPP. It would cost you 23 IPP to purchase a fleet carrier and an escort carrier to carry the same amount of planes.
Advanced Submarines are a favourite of mine for the US, Japan, Germany, and ANZAC. They’re harder to kill than regulars subs and they have the same punch as a Destroyer. ANZAC can’t afford a big navy so they are very effective working as hunters against Japanese navy stragglers. Japan really has to think twice about splitting small groups of boats off for amphibious assaults and such. Carriers are especially vulnerable because if you nail one with first strike you may sink their planes if they don’t have a landing spot available. Lend leasing them from the Americans is a great idea because you won’t need to develop the tech with the UK. Two birds one stone.
I have a 16 sided die and we roll it for each country at the beginning of the game for each nation. You get whatever tech rolls on the die. It gives you a fully researched tech for free so you get some weird marriages of nations + tech. You wouldn’t normally try to develop attack transports with Russia but they sure work great dropping off heavy tanks in Sweden.
Never underestimate the power of a long range jet fighter for any nation on the board. Now that you can scramble them into adjacent territories
they are a killer with an air base, escort bombers like a boss, or pound anyone’s navy.