From the players side it must be said, that you managed always to bring out a more or less stable version, which is playable, even with bugs. 0.8 is a small exception in this line, but there where always ways to go around the bugs.
And the problem is really the time needed to test some stuff. Well, I've seen
some projects going totally down the drain (in one I was, we just made the curve in the last second), because they didn't make their software testable. And testable means much more than unit-tests.
What do you think about
automated integration tests? I mean you have unit tests, but I don't know, if you have also integration tests?
If not: an integration test for Factorio would be
like a special map and the character runs automatically around like a robot and does things, and if the expected behavior comes out, we have tested some functionalities. Simple tests like there must be 1000 iron in a chest or a train is at station B or or more complicated like when pressing here there must happen that. Or, or, or.
Which leads to much faster testing, which let you find earlier the bugs, which leads to faster programming.
The point, why I'm diving into this is, that I mean, that testing like so isn't currently practicable. Too complicated.
What's needed are hundreds (!) very small to very large "scenarios", which tests very different aspects.
And thinking of that brings me back to blueprints! Because I think that you are working on that and this would be a great technique to create all this small things and repeat an existing test case over and over, with this small but important differences to test all this border-cases. Think for example to the inserters, there are four orientations, and a dozen types of entities you can pick up and place. Not to think about priorities. All that should be tested in a way, that even a minimum number of change in throughput or behavior should be recognized.
Or in other words: if you built blueprints into Factorio, make it useable from very small to very large scales. Large: 200x200 tiles minimum. Because then we (the players) can provide you with maps, where one aspect of Factorio is tested over and over for hundreds of cases. Make it possible to load a bunch of maps and letting them run for some time/event. Catch the the test-results. (If you would built all that also into the map editor, you lay the base for modders to create game scenarios!)
Make it possible to built maps from some smaller maps or blueprints. Let's having "layers" (e. g. one layer for the ground, one for the entities and one for the items), so that I can "reset" a map by loading the item layer of an older save, even, if the belt layout has been changed.
And half a dozen of more ideas...

I stop here, because my experience says, that you might already do such stuff and that it is too long otherwise. But I'm willing to explain, if there are questions.
And in danger to repeat me: the big advantage would be to have tests, repeatable tests, which will help you to keep the stability (fix one, break another) and to test new features much faster.