My impression was that the lesson learned from the space shuttle was that it was nearly or actually cheaper to build expendable rockets from scratch than to inspect and maintain reusable spaceplanes.
Now, the atmosphere isn't perfectly gas. There a little bits of dust and sand about, even at high altitude due to forest fires, volcanoes, sandstorms, etc. Not to mention little bits of ice that the engine cooling will produce itself. We know that turbine compressors erode. That's a fact of life. But, it's fairly easy to replace compressor blades.
From what I saw in the video, that heat exchanger seems to be both very fragile and very complicated. But all of these little bits of solids are going to be whirling around this engine at tremendous speeds. Thousands of tiny tubes that are delicate even to touch? I have a feeling it's going to get dinged up badly if it's ever put into use.
If you want to get airliner-like costs, then don't you have to have airliner-like maintenance and turn around? I didn't hear anything in that video that addressed ruggedness, ease of maintenance, or improving turn-around time. That concerns me. I'm rooting for SSTO, but I'm afraid that these engines are not going to be as reusable as they think.
My apologies if this has been brought up before, I haven't read the entire thread.