There now have been many games on Stadia that ONLY later we find out the devs aren’t supporting the game with DLCs or even updates. Google really need to address this.
Depends on which games. Many add-ons for games are actually cheaper on this platform than others. Take steam for example. The DBD add-ons are pricier there on occasion than here.
There are definitely some games that don't have all the add-ons. But that's up to the developer. Not stadia in the grand scheme.
As for DLC and updates, at least, when they ARE included, they're applied immediately. No need for users to download, install, or restart. Just launch the game, and it's the updated version. Stadia = no thermometer bars ever.