This would be for a gravel bike. What’s the benefit of a carbon stem…other than possibly lighter weight? I can see (I think) benefits to carbon bars. Is there a similar benefit to a carbon stem? Or is a carbon bar with an alloy stem fine (I know it works…just wondering about theoretical benefit).

I like carbon bars, but alloy stems are fine. Carbon stems can be very spendy and, from what I've read, are often flexy. Personally, I'd only do a carbon "stem" if I were doing a one-piece bar/stem (or if I found a great deal on a great carbon stem that tested well).

Carbon bar pros: lighter, possibly more vibration dampening, more shapes/sizes/contours available. Cons: more expensive, possibly could be damaged in a crash with damage hidden underneath bar tape.

Carbon stem pros: possibly more vibration dampening. Cons: not any lighter, more costly, and most people don't bother.

Personally, I prefer aluminum for both. I suspect most "serious" cyclists prefer Carbon bars and alloy stem.

Alloy stems are fine, and mostly weigh about the same as carbon ones.

In terms of vibration dampening. I have a carbon stem because it was almost free with the bike, I can't feel a difference between it and any other stem I've used. Most of the CF handlebars have felt more comfortable to me on rough pavement, not so for the stem.

I can’t imagine going with a carbon stem with my carbon bars unless the combo of both allowed me to go pseudo internal cabling

Even on a carbon bike. Most clamp force parts I just prefer to be metal. Especially after a crash I trust metal more. I like Zipp service course parts for these applications. Really well designed imho

This is one of those upgrades that you hear very conflicting advice. Numerous people love carbon bars and say they are very noticeably more comfortable. About as many people say they can't feel any difference. My guess is it just varies by person and bike. Makes it hard to decide to spend the money because who knows which you will be. My personal take is it's not worth upgrading for upgrading sake, but if I decided to replace my bars for some reason I would probably go ahead and try carbon.

This^
I have found carbon bars to give a significant advantage in vibration damping on both mountain and road bikes. My current road bike has a one-piece carbon bar/stem, which feels great. Easily the best road bar I've ever used. If I was using a separate bar and stem I wouldn't bother with a carbon stem, but I would definitely use a carbon bar.

+2

IMO carbon stem/bars are things I don't need. But, if it speaks to you have at it.

I have the same preference and also agree with your recommendation as I am oddly fond of my Service Course SL with the Quickview faceplate computer/light mount.

Carbon stem seems to be way into weight weenie/carbon bling territory. Other than shaving a few grams off your bike and wallet, I can't imagine there's any real benefit. If a stem is flexy enough to provide any actual "compliance" benefit, then it seems like it would make for some super weird handling.

I do like carbon handlebars and have a very fancy carbon bar on my CX/Gravel bike. I run it with an alloy stem.

If it's for gravel, I'd get sturdy triple butted 7050 alloy drop bars (like the Ritchey WCS bars), stem (like 7050 alloy Uno stems) and use titanium bolts. You'll enjoy weight savings and durability. My gravel bike gets a bit more abuse, so I'm okay using alloy. I also like carbon drop bars since they can be designed to be more aero, but they still flex and wide gravel tires can absorb much of the road vibration.

There are often tangible benefits in things we don't "need". For me carbon bars are definitely in that category. Carbon stems maybe not, although I appreciate the benefits of integrated bar/stem combos.

What is the tangible advantage of CF bars? Maybe you can change my mind.

Having arthritis in my hands and wrists I am very sensitive to vibration and have found in the models I have tried better dampening in general with carbon bars and no difference with stems. Carbon passes the same safety standards as alloy bars but if you CX or MTB I would not use carbon in that a damage issue with an alloy bar is obvious (unless under tape corrosion) and may not be as obvious in carbon.

Might as well discuss galvanic corrision. Is it a problem with carbon stems/handlebars?

Depends.

Galvanic Corrosion of Metals Connected to Carbon Fiber Reinf (corrosionpedia.com)

alloy stem feels stiffer

Carbon bars damp out a lot of road buzz compared to alloy bars. For example I currently have 2 very similar road endurance bikes (Giant Defy and Canyon Endurace as per my sig below). My Defy has alloy bars and my Endurace has an integrated carbon bar/stem. While both of these bars are endurance/comfort oriented, the Carbon bar on my Canyon is far better at isolating my hands from road vibration. I notice it most riding on chip-seal roads. Even with thicker bar tape on the alloy bars, it's not as good.

The other issue I have with alloy bars is corrosion. I used my Defy on a turbo trainer over winter and sweat pretty much destroyed the alloy bars. While that was my fault and could have been avoided, it's one less thing to worry about with carbon bars.

Thanks for the explanation

The stem is so short and stiff and thick I doubt it can do much to dampen vibration.... It's totally unlike a handlebar.

That said however, I have a carbon stem. It's main advantage is it's nice to look at!

I would agree with this. For me it also makes sense to integrate the bar and stem when working in carbon (presuming you are okay with the fixed angle). Removing the bar/stem interface has a lot going for it in my book when dealing with carbon.