So, the biggest difference in quality is the steel and hardness.

My question was rooted in how you would know the quality/hardness of the steel at time of purchase - especially as a (fairly) layperson.

I think my set is dewalt. It included taps, dies, and drill bits for M3-M7. I don't think I've ever used the drills, and the have gotten a lot more use than the dies, but I think I've used almost everything in the kit more than once. Before kids, it was cars. Post kids, it's mostly cheap Chinese furniture my wife buys that I have to chase threads in to get it to assembly well.

Also, for the record, you can absolutely tap plastic for a reasonably strong thread

I've had pretty good success sizing 3D printed holes to be interference fit. That's how I designed/printed the bed leveling thumb wheels for my i3 clone. They backed off far less frequently than the stock metal once.