Slime tubes worked great for me when mountain biking. Think I pinch-flatted once, though, and that was a mess on the trails.
I think if it's a high volume/low pressure application then slime (or any other sealant) inside inner tubes could work well. I am skeptical about skinny tire/high pressure use cases, though.
I've had ok success with tire liners on commuter and road bikes, but did manage to get a puncture once as well. Eventually, I decided that the extra weight and hassle of installing them properly led me to give up on them. I'm a low mileage guy who mostly ride on smooth roads now, and my gravel bike is set up tubeless so I haven't had punctures in a while. If I really wanted to guarantee flat-free riding on roads with a lot of urban debris, I'd probably go with a tubeless setup or tire liners with inner tubes.