Cons? Unless you yourself are a TDF level racer you probably won't notice any. I have a vintage steel frame built up with modern components (5700 105) that is around 20 lbs. Not light by modern standards but obviously a similar frame built with lighter components would be lighter. If you have enough money to spend you can even get a 13.5 lb steel bike:

The lightest custom racing bicycles | Lighter than carbon fiber | The Steel Rodriguez Outlaw


I also have a lighter, stiffer modern carbon bike. I can tell the difference as in the steel is much more comfortable and a little flexy but according to Strava my times, avg speed etc are almost identical on the same routes with both bikes. Some days I'm faster on the carbon and some days on the steel and that is all based on me not the bike or material