Subs are crap and convoy raiding is pointless.
Except for when they are not, like you admitted in the last thread we debated the topic. They have many times as high detection values as any other fleet ( if you get doctrines ) and are needed if you want to be able to stop invasions after they have been launched ( which is not a big concern vs the AI, but very much so against human players ).
They can also raid places the AI never bothers to patrol and a single submarine can sink tons of divisions or hundreds of convoys when put in the right place.
Cruisers are bad but destroyers are even worse. If you need to kill subs then just build a carrier or two. Don't throw away your dockyards on crappy destroyers.
If your enemy put small fleets of 1-3 subs per seazone your screwed with the approach of trying to use Carriers alone to kill them. You will never have as many Carriers as you have seazones to protect, and if you split up your Carriers in smaller fleets they will get wrecked by the enemy main fleet. Even if your Carrier fleets can eventually find subs your enemy can build new subs faster than you can find and sink them.
Naturally the AI won't use their subs properly, so you don't have a great need of DDs in singleplayer, but if you play a multiplayer game against someone using submarines properly then trying to fight them with Carriers alone simply won't work. Especially not when your forced to fight subs in range of enemy land based air ( like of the coast of German occupied Norway to protect allied trade or Lend Lease to Soviet ). Putting Carriers there permanently is asking to get them bombed and sunk, while DDs have low enough hitpoints to not trigger massive airstrikes and are fast enough to repair + cheap if you get unlucky and lose them that they can effectively fight the subs there.
Mechs aren't worth it until you're awash in IC. I often put them on modern tank templates to boost their stats a bit, because I have enough IC at that point to splurge.
They are decent at increasing the hardness and armor rating of tank divisions which allow them to take less damage, but I agree they are not cheap. They are super cheap compared to using Fleet Carriers for Anti submarine work though...
Military police are actually net negative in terms of equipment. Signals is just not worth the research..
Signals are awesome if you care about making sure your divisions are not outnumbered 3:1 or worse and as a result get wrecked in combat. They can triple the chance of reserve divisions joining combat. Anyone not going mass assault should consider using them in their quality divisions.
Heavy tanks are worthless with their slow speed, high cost, and the fact the AI spams AT on all its divisions.
SPAA can be useful if airport space is limited, e.g. when you're establishing a beachhead in the early days of invading a late-game USA. TDs are worthless.
The main job of Heavy tanks is wrecking the enemy Medium tank divisions, so against the vanilla AI ( which either don't build Medium tanks or can't use them properly ) they will always be of limited usefulness. You should still be able to surpass the AIs piercing from AT comfortably if you want to do Space Marine divisions with +5 armor variant 1943 Heavy Tanks or HTDs to chesse the AI.
For quality main tank divisions, sure SPG is better. However for fast Motorized/Light Exploitation divisions, especially if your short on tungsten or run concentrated industry Motorized Rocket Artillery do a great job as cheap and soft battalions that boost firepower.
Normal support rocket artillery can be very powerful if you go Superior Firepower doctrine, right branch.
AT is very suboptimal compared to tanks/planes.
If your not using AT or AA in multiplayer and your enemy makes space marines your dead. End of Story. Against the vanilla AI I agree you can get away without using them, but that does not mean it's worthless. If you play against harder AI mod that gives the AI competitive templates I would not advice skipping either AT or AA so your divisions have no piercing at all.
Towed AA is crap compared to SPAA.
Still it has about twice as much Air Attack per MIC cost as SPAA does, cost no resources but steel and gives you enough piercing to defend vs light tanks. I would say it can have it's uses. Per battalion the cost difference is even larger ( comparing to support AA ), it will be a quite expensive project if you want to arm all your infantry divisions with SPAA.
Heavy fighters can be good late-game when airport space becomes a bigger limiting factor than IC.
No Heavy fighters become worse the later in the game you get vs light fighters. Only the 1936 model can really compete. Heavy fighters lategame are only good at 3 things):
1.) Kamikaze since they got 50% higher Naval attack than light fighters.
2.) Gaining air superiority in large ( pacific ) seazones where light fighters have no/little reach
3.) Shooting down unescorted Strat bombers.
Strats are net-negative. You blow up the roads your tanks need to blitzkrieg.
So that's why they are banned in most MP games? Because they are net-negative?
