Having built a number of small gas turbine engines, you are not wrong. Small ones have real issues with acceptable performance. Clearances are a big part of the problem. Williams tried a 3 spool turbofan for the Eclipse jet. Even using one piece bladed disks and ultra tight clearances, Williams was never able to achieve predicted power or TSFC (economy) . Pratt came to the rescue with conventionally sized parts in a compact package.
Aviation is full of bogus claims and a harsh reality. That reality being Range, Speed and Altitude are almost mutually exclusive goals. The smaller, the more elusive those goals become.