Do web browser emulators and simulators actually work?

I know that I can use online tools to emulate and/or simulate browsers and devices for webpage testing, and I know that the question of defining what emulation vs simulation is has been addressed on Stack Overflow (How do emulators work and how are they written?).

But, do they actually work?

Are they reliable? And if they are reliable, which online emulators/simulators are best?