Great, congratulations. Firesteel requires a lot of experimenting before you get to know its benefits and also limits. I reccomend buying one piece just for training and experimenting. Like you have seen 95 % of the success is the tinder. This canīt be stressed enough. I have learnt it the hard way too.
I have seen a lot of complaints from people on the Internet about ferrocerium being too difficult to start fire with and that you would be better off using a lighter but they miss one part of the equation - the tinder and how to prepare it.
One should also keep in mind that with firesteel you trade easy-of-use for reliability.

I would say once you know and have/carry the right tinder itīs really the most reliable way to start a fire.

One tip (maybe you know this already): try to rip/mill the birch bark into the smallest pieces possible. This applies to almost any tinder. Generally the fibrous stuff works best. Birch bark is great because it burns even wet.