Keep in mind you're one individual - for every bad experience there are others (like Reddit) who have had good experiences and used it successfully; it's all down to the skill of the individuals to ensure they've configured it correctly, using the right data structures and so on.

If you had $10 million, you could pay a team of 50 people who work 24/7 (in rotation) to do nothing but find redis bugs and fix them, and make redis the most stable software on earth. Obviously the more resources you throw at something, the more stable it becomes, this is a software coded by 1 guy, and you're surprised it may have a few issues?

how is paying for the same source code

Go through my comment, and quote where I said "pay for source code". The source code is free; you're paying for continued support and maintenance, which may include hiring those 50 individuals who do nothing but bug test.

Think of it similar to video games testing, trying to fall through the map, that's what testers are looking for, and is a pure cost to the company but in return get a more stable game.