I see Pro's mentioned ... but not any cons? Input from you guys?

I’ve never run one but my guesses would be durability and reliability. If you have too much gas/not optimized for the BCG it would bang it around more than a standard weighted BCG. Same with reliability, more fine tuning needed.

Messes with the timing of the system, same as using an incorrect buffer weight/spring. I am in no way an expert on this, just relating my understanding.

If the BCG accelerates too fast it is ripping the case out of the chamber before it has cooled and contracted at all. This puts extra stress on the extractor. Accelerating the BCG excessively fast also puts more stress on the cam pin and bolt hole that the pin goes through.

Accelerating the BCG too fast can be caused by a couple factors.
-too light of a system (bcg+buffer)
-too short of a gas system
-too large of a gas port

If you read between the lines here you can probably understand that some systems with short barrels are inherently a compromise on some of these.

In theory if you properly designed the whole system including downsizing the gas port then you could probably make a lw BCG work correctly. The short answer is that if you take a system designed for a standard weight BCG and drop in a LW BCG the system will not operate correctly.

They shouldn’t be used without an adjustable gas block

I run them in over half of my guns because I like lightweight guns!  In my opinion and experience there is not much in the way of cons using a lightweight BCG as long as you don't go crazy light and stick with a steel bcg around 8 or 9oz. If you go  lighter than that you have to do more to make things work together and yes it may become less reliable!  Yes you will want to use a adjustable gas block or gas key with any lightweight BCG!
Just my opinion, good luck!

They were originally designed to run lightweight bullets at the fastest possible cycling rate for competition against a stop watch. I have no idea why anyone would want one outside that narrow window.

Assuming it's gassed properly, I imagine reliability would be affected (less mass for loading/chambering)

Faxon makes a full auto profile lightweight carrier. Master of Arms makes a pretty cool one tool that has a deadblow feature.

All in all, the biggest issue that could be found would be bolt bounce imo, but I haven't really seen that myself.

Bolt bounce is only an issue if you are shooting full automatic, have you been shooting full-auto?

Lighter carriers reduce the operating margin.  With the normal USGI system, the system will digest a wide range of ammunition in a wide range of environments without any adjustments.  With a light carrier the system is more sensitive to the port pressure.  You might find yourself fiddling with the gas more if you are regularly switching between regular high pressure stuff and low power stuff...

Thanks guys for the info. I'm trying to decide the parts for my next build. I'm thinking the Failzero lightweight bolt carrier group. I've already got the adjustable gas block on order as well as the Nomad Ti suppressor. I've already got the Proof Research 12.5" 300 blackout barrel. I only plan on running one type of subsonic ammo.

Because less reciprocating mass equals less felt recoil and faster follow up shots. Set up properly with adjustable gas, they are just as reliable as any other carrier

Pros of lightweight BCG:
* Less recoil from reciprocating components
* Faster rate of fire
* Less gas required for functioning, which means you can reduce the adj gas block opening, allowing your muzzle brake to be more effective at reducing recoil, and also less carbon fouling/less frequently needed cleaning
* Overall rifle is lighter, allowing you to carry your gear for a longer duration of time before fatigue sets in, or to carry more gear for the same time
* When firing unsupported, lower weight allows you to remain on target for longer durations before fatigue sets in

Cons of lightweight BCG:
* Lower mass BCG has lower momentum, which increases risk of bolt not closing all the way when there is extra friction, such as releasing the charging handle with the BCG, or not pulling the charging handle all the way back, or too much fouling in the chamber/not enough lube, or cold weather conditions (stronger recoil spring may help with that, although that increase in velocity can potentially cause other issues as well)
* Higher velocity BCG increases risk of slam fires (titanium firing pin supposed to help with that)
* Faster unlock time, which in theory means higher chamber pressure and more port pop when used with suppressor
* If BCG is moving too fast, there is potential for outpacing the magazine spring, resulting in failures to feed (this is unlikely to occur from simply using a lightweight BCG, it would only occur if you are doing a lot of other race gun modifications in combination)

In my opinion, the pros of a lightweight BCG far outweigh the cons, with the potential exception of when you want to make a very quiet suppressor host