Kegs are clearly the best way to do just about anything. It doesn’t matter if your process is a secondary ferment, a bright tank/clearing, or carbonation and serving. It’s a sealable stainless vessel, so what’s not to love?
If you are taking it out of primary, your fermentation should be complete enough that the yeast won’t be generating much gas, so no airlock is generally required. As for an “airlock” when needed, just connect some tubing to a gas disconnect and place the end of tubing into your choice of water, vodka, or sanitizer and you have an airlock that will never have any issues and can be used for primary fermentation too if you use the kegs for primary as a lot of us do. You also don’t have to try and rig one of those little plastic things or modify the keg or connectors.
At the end of the last fermentation stages (primary or secondary), I prefer to chill the last fermenter to get the maximum clarity for a few days and then rack to a serving keg to carbonate and serve.
I rarely carbonate using sugars, but if you choose that route with kegs, just ensure that you use a keg that you know is capable of holding a seal with little or no pressure. I always use a pop of C02 to seal the lid when naturally carbonating for insurance and it works just like a big bottle of beer. If it fails due to leaks you can always force carb