What they assume is that they will be able to achieve these goals while maintaining the reliability performance the same as a traditional AC engine based on the reliability performance of the machine the engine was designed for. It is a flawed assumption, but the promises usually overwhelm the desire to critically challenge them.
Not really flawed as many people you probably don't know about have shown. Here is a good example (skip to page 27 and pages 29-33): http://www.glidingclub.org.au/Documents/Airflow/Airflow 2011-3 Spring.pdf
http://www.glidingclub.org.au/members-only/aircraft/etug/
The 200 less cubic inch LS1 did this demanding job much cheaper and better with far less maintenance than the 540 Lycoming. Previous to this, a V6 Ford also did it cheaper and better, paving the way for the later V8 conversions. http://victoriancollections.net.au/items/510a18022162ef11145dba31
I could list a bunch of others but I'd be repeating myself.
Yes, not something that anyone can do but it's been done more times than most of the readership here on VAF know so it's certainly possible with the right background.
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