Sorry I missed your question… the one legit question this thread has had, so far (embarrassed)
It’s tough for us to say, really. It’ll depend a lot on your background knowledge and how that translates into the care with which you use them. What we CAN tell you are some of the worst-case scenarios (fire, venting, and theoretically, small explosions). Moreover, we can tell you that these events are RARE, and shouldn’t discourage you from trying cells if you exercise good safety precautions. The cells themselves are less likely to be the problem. If you’re prepared to wait a little bit, I hope to have this guide completed before the end of the week. That should answer a lot of your questions.
As for the quality of the cells, the experience of forum members is that with the cheapy cells, you sometimes get duds, have cells that are nearing the end of their life expectancy, and will in all likelihood, have less than the rated capacity (sometimes MUCH less). I probably did worse on my first purchase, and I’m still here, so don’t worry too much. I, personally, do not have experience with the products from this seller, so I cannot say what you’ll end up with.
(can’t sleep… clowns will eat me)
edit
+1 on checking to see if they’re protected. Don’t use unprotected cells in multi-cell flashlights. Also, ensure that both batteries being used in the flashlight are charged to the same voltage; if there’s a difference, charge them to match before installing.