For my road bike, I am quite happy to use a CO2 since a single one will get me out of trouble. Long rides I'll take 2 but I've never used 2 on a ride.

For MTB, I've started carrying a hand pump. Yep, it's slow but the progression of a MTB flats takes more.
1. Get leak that sealant isn't sealing
2. Plug and pump up
3. If it doesn't seal, add multiple plugs to same spot and pump up again
4. If that doesn't work, pull off the tire and boot it and put in a tube.

98 times out of 100, you're done after step 2. In fact, most times you don't even know you've punctured until cleaning it up many days later. If it does progress to a situation where you do finally put in a tube, a single CO2 or even 2 is unlikely to get the job done.

To me, this isn't all that different than a battery operated pump. How many tires can it fill, how many times can it top you off before the battery is dead? Given the extremely infrequent use of pump while out riding, who knows how many times if forget to charge the battery. For a different reason than CO2, there's still a solid chance you'll run out of air when you really need it.

I'm sure the pricey Silca and Topeak models are better than the junk ones. I'd probably be too nervous and still carry a hand pump though.

I'm currently still carrying CO2 only on most gravel rides, but for rides further off the beaten path when I'm wearing a hydration pack, you better believe there's a hand pump in there.