Armor is much better balanced in this game than Payday 2, with all of them seeming equally powerful. The standard lining grants you a 4th down, while the heavy ballistic lining takes you down to 2 max. The light ballistic lining and medium ballistic lining both keep you at 3 max downs.

Lighter armors have better movement speed and armor regeneration speed, while heavier armors offer more absolute durability. Because heavy armors take longer to regenerate, the perk Plate Up in the ammo tree is great on them.

I personally prefer the standard lining, or maybe light ballistic lining. I don't like how slow you move in the heavier armors, and how long the armor takes to regenerate. It feels too reliant on Plate Up. Overall I'd say the heavier armors are easier to use, but not necessarily better. Just easier.

The deployables also seem much better balanced than in Payday 2. Doctor bags no longer innately restore all your downs, so you have more choice on what to do.

The armor bag is good at restoring lost armor, obviously. If you crutch on it and don't manage your armor properly you might feel like you need a ton of these, but even on OVERKILL you don't need more than 1 or 2 (assuming they're upgraded). Just deploy them in a good spot, don't deploy them before you need them, and don't use them when you don't need them.

The medic bag is good at both restoring lost health and more importantly working with perks. The perk Last Man Standing gives you 10 seconds of invulnerability when your health would run out, but requires you to recharge it by either downing or restoring health from a medic bag or first aid kit. The perk Field Surgery makes medic bags give you an additional down. Because you don't need 4 armor bags, I find it helpful to have at least one heister bring a medic bag. Even if someone is using a max armor build Last Man Standing is still great to have for emergencies.

The ammo bag restores both ammo and throwables. With perks it can also grant a 20% damage bonus for 10 seconds. With max investment an ammo bag can restore 30(!!) grenades, grant 60 seconds of increased damage spread out over 6 uses, and restore a ton of ammo. I'd say it's less important than armor and medic bags, but if you already have other people bringing those the ammo bag can be very helpful.

I haven't actually tried the sentry out very much. Can't say anything about it, sorry.

My favorite build relies on managing armor, health, and consumables. But outside of there being infinite hostages managing resources is very important. Regenerate as much armor as you can, only take from consumables when absolutely necessary, etc. Spamming Last Man Standing is funny and useful in dire situations, but constantly trying to use it as your only defense throughout the entire match is going to burn through too many resources and leave you with none later on (Unless there's a ton of hostages, then the resources are so abundant that it's basically just permanent godmode as long as you keep playing the minigame).

The vital skills in this build are Medic Aced, Combat Medic, Last Man Standing, Manipulator Aced, Silver Tongue, Negotiator, Menacing, Escapist, Move & Cover, and Tactician.

Medic Aced and Combat Medic make it so that starting to revive a teammate grants Grit, and 40% damage resistance. Once the revive finishes both of you are invulnerable for 5 seconds. This is huge, and lets you do a lot of things. Safely revive the entire team even in the most dangerous of areas (as long as you can get the first revive off), revive someone then facetank a dozer to death, even just revive someone then get to safety while invincible.

Manipulator Aced, Silver Tongue, Negotiator, and Menacing make it so that trading unemployed civilians gives 3 first aid kits, while everyone else gives 4. You can dominate cops to have an endless stream of faks (that require assault intermissions). If you trade a lot of civilians and just litter faks you basically are immune to everything except Cloakers if you're within 10 seconds of the faks.

Move & Cover and Tactician make it so that nonlethal shots on enemies are extremely likely to give you Edge, and mantling will replace Edge with Grit. This makes it very easy to get both Edge and Grit, and Grit is vital for the next skill. Sprinting for 3 seconds to get Rush is also very helpful, but not a necessary part of this build. Escapist is necessary for Move & Cover, but not necessary to the build (although its perk of easy to proc Rush is very helpful).

Last Man Standing makes it so that as long as you have Grit, which is easy to proc from Move & Cover, you will not die when your health runs out. Instead, you become invulnerable for 10 seconds. It prevents you from getting Grit until you use a medkit or first aid kit, or get downed. You can use a medkit or first aid kit while still invulnerable, then obtain Grit while still invulnerable, and chain the buff together. On a different build this would be great as an emergency last resort thing, but this build creates so many first aid kits that it can often be used as the main method of surviving damage. Just be careful not to overuse it and waste resources.

This is things I've noticed that the game doesn't explicitly tell you, either implying or omitting them. Let me know if you have anything to add to the list.

Any perk that says you gain a buff will only grant it if you don't already have it. If it grants it regardless of you having it it will say "gain or refresh".

Manipulator Aced and Silver Tongue affect both the value of hostages for more time at the start, and how many first aid kits they give. This allows each hostage to give 3 or 4 first aid kits.

Master Trader doesn't apply to bots. I think this one is a bit obvious, but just in-case anyone was wondering it doesn't.

Plate Up restores the red portion of armor, not the entire chunk. I also think this one is obvious, but I've seen people confused by the wording.

Steady Hands and Field Surgery seem to apply to first aid kits. I haven't tested it, but after unequipping them the first aid kits seemed to heal way less. I don't know if they restore downs.

Cutting Shot makes weak weapons not so weak. It turns the M16 from a 3 headshot gun into a 2 headshot gun. Revolvers can also onetap, but I'm not sure if it requires Cutting Shot or it's just Edge.

Overcooked applies to flashbangs, causing them to instantly detonate which is pretty helpful since they can't normally be cooked. I haven't tested smoke bombs but I'd assume it also affects them.

The doctor bag does not innately restore downs as implied by its description, and requires the skill Field Surgery to restore 1 down.

Enemies can only be made into human shields when unaware, stunned by a flashbang, or dominated with Menacing. Because enemies are only unaware before the alarms are raised, the CQC tree relies on either of the other two methods to make use of.

Human shields will prevent all cops from shooting at you while you're facing them, but they will attempt to approach you and bash you with a melee attack. If they succeed, which they can do even from behind, you will be stunned and the hostage falls to the floor.

You get increased money and weapon experience the further you get in a heist before the alarms are raised.

Grenadiers don't throw gas grenades near hostages, and SWAT will use pistols instead of shotguns.

Cloakers, Tasers, and Gas deal damage to health, ignoring armor. Their gun attacks still damage armor normally.