I have had to use a tube twice on my gravel bike, but both were my fault. Let the sealant volume drop too low in the summer once (it evaporates faster with higher temps), and didn't have enough left to seal a puncture that would have been fine with a plug. Also, while listening to music, didn't really notice a puncture that was spraying sealant everywhere until the tire was almost flat, and the sealant was all gone (w/o headphones or high wind noise you can usually hear a major puncture).
I have also learned the hard way in a race that it's best to just spend the $40 on a Dynaplug racer tool that works REALLY well, and quickly, and to just keep it in your jersey pocket for quick access. Even if you don't care about the stop time, you want to get a puncture plugged as quickly as possible to minimize sealant / air loss. A co2 should snap the tire back on the rim if it un-hooks a bit from total air loss, but there are some situations where it won't, and there's another time you need a tube.