So the bad one is #1. It came out of a Doyo reel and is not the first one I have seen like this. If you look at the teeth in the upper right corner (2 o'clock), they have small scratches in them. This might be ok but the pinion gear is damaged. If you zoom in, you can see the chip out and also the inside of it is grooved. This reel is 3 years old.
#2 is from a Metanium MG7 which is the original from 2007. The guy it came from lived near Lake Fork and it was his primary reel. It was serviced each year though but it has seen more big fish than I will see in my lifetime. This reel is as smooth as butter but is not as smooth as my Core. You can tell the difference between the brass gear and the aluminum.
#3 is from a Shimano Antares 2012 that was used in saltwater. The reel itself was in pretty good condition and was taken care of but the problem with it wasn't the gear set, it was a bearing. What you see in the picture is what it looked like after it was cleaned and polished.
#4 is from a Banax reel. This is sort of a tough one. I have found little wrong with the gear set itself but there is some play between the drive shaft and frame. Also, there is some wear and play in the pinion grove that fits the pin on the spool. It will make a clicking noise occasionally. Under load the retrieve is not the best but just reeling a bait in, it feels fine. This reel is around 2 years old.
To me, the gear thing is over blown. Brass, I think, will soften the feel of other problems a reel might be having. On the other side of it, I can't say Aluminum transmits the feel of the inside of a reel more. There are just too many things that could be causing issues and the pinion and main gear should be the last on the list to check for problems. Dirt and gunk that get into reels cause more issues, to me, than anything else. Keeping a reel clean and serviced does help a lot.
Also, I have only seen one gear set from Daiwa that was bad and that was years ago. Plus, I didn't have any to show at this time.
Adding one more disclaimer, these are just observations and do not represent each manufacture individually. I own or have owned reels from each of them and have had both good luck and bad luck with each.