Firstly, I use 2 Piece Borg (Engines and Console) coupled with 2 Piece MACO Mk XII (Shield and Deflector) on my Engineer/Advanced Escort build.

Why do I do this? It's pretty simple.
I do not benefit from the minor Shield Heals when I can have more reliable shield heals as an Engineer.

I am a Eng Captain. That means I have amazing Hull Heals. (Quick Fix, Eng Fleet). This also means I get EPS Power Transfer III (Gives me 75+ Aux). Escorts mean I get 2 Tac Teams. Throw in some Shield Heals And I can tank nearly anything PvE can shoot at me.

Now the big thing about the MACO Deflector is that it gives bonuses to Shield Emitters (Boosts the Shield heals I already use), SIF (So I heal faster), and Shield System (Better Shields!).

Also it happens to have Starship Electro-Plasma System which even more helps out my abilities (EPTS / EPS Transfer)

Out of this the Borg Defender gives the SIF Bonus. I don't need everything else the Borg Deflector gives (I don't need even more shield heals that are at a 10% chance. Those won't save my life.)

I used to use the standard MACO shields and 3pc borg like everyone says to do, but now I don't even use 2pc borg. I switched to 3 pc MACO because even though the set bonuses aren't hreat, their stats are better then the borg deflector and engine. I find it works better for me to have consistently better stats then have the small chance of getting a hull or shield heal. I use it on my tac in an escort and find I don't miss 3pc borg at all, its way overrated.

How Do those MACO Engines work while Not moving (Turn Rate wise) / In reverse compared to Borg?
They look really good. I might have to get them (Probably impossible as I never can seem to do CSE).

The MACO deflector is unquestionably superior on cruisers, and its stats are generally just flat-out superior to the Borg one full stop unless you're running a specialized build. That said, I've tried every possible combo you can think of on an escort and the amount of survivability you lose without the full three-piece Borg bonus is massive. The proc doesn't sound like it'd be a lot but it really, really is, it's hard to explain unless you feel it in practice.

Haven't tested it with a parser, but I really don't notice any loss in survivability without 3pc borg. The better stats of MACO items seem to make up for it. Been using it for a couple weeks now and have no plan on going back.
3pc borg is just something I will use on characters that haven't got full MACO or Honor Guard yet.

I hate to break this to ya but the proc really isnt that important. I've run both 3 piece with shield and full maco, I find survival rate is better for a tac in an escort with full maco. that said, that meens this is amped when you add in a sci toon or a a massive amp with an engy. I like you thought i would miss the borg 3 piece proc, but really i never saw it proc but a couple times in a fight. I think maybe a 2 and 2 combo is good and effective, you still get the main 2 procs from both sets. i dont think the hull proc is worth the added piece though, imho.

On my char with the full Borg set, I love it! I get 1-shotted more than with Aegis, but it's pretty good vs. other stuff. When my Sci char uses it she gets around 1440 per cycle of the hull heal.

You don't see the proc because you don't run a combat parser. What happens on screen is rarely reflecting what's actually happening behind the scenes, often the Borg procs don't trigger visually properly, same with the invisi-STF torps. Part of the Borg set bonuses aren't even "procs" as such since they passively boost hull repair.

Even just doing mathcraft, full MACO has terrible synergy with anything other than maybe the Patrol Escort (old Fleet Escort). You're just not using Eng/Sci heals often enough to justify the tank stats you'd get from the deflector, and some of the stats are almost useless anyway (Inertial Dampeners, Graviton Generators on 90% of escort builds). I'd almost prefer taking 2-piece Aegis if I wanted to be a tankyscort, Aegis engines dump on MACO.

The thing with the Borg procs as well is that it means you're not nearly as reliant on BOff powers to survive. If you get hit hard and your EPtS or whatever is on cooldown, you're likely in trouble, while if you have Borg gear you'll likely be just fine.

To everyone who doesnt see the proc. well its PvE, PvE in STO is rediculusly easy. If u pvp however. Ull realzie MACO 2pc does NOT justify loss of the proc. In pvp ull notice the borg proc will save u 9/10 times. In pve, ur better off with a heavy dmg build, and heals dont really make much of a diference, You wont notice because NPCs hit hard and slow. So a 10% proc happens less.

But this is the PvE boards, if I wanted to PvP I would just use my Tac-BoP.