I would hope all of the above is considered by Aero Companies and Regulatory Agencies prior to STC approval
The design requirements which must be shown are the same for the aircraft, and the engine respectively, regardless of what type of fuel is being approved.
Back in the day, 100/130 (green Avgas) was an
alternate fuel for those aircraft designed an approved for 80/87, it was certainly not choice one for those low compression engines. When the two of those fuels were discontinued for the single 100LL (for the Avgas suppliers convenience I expect) somehow 100 octane leaed Avgas became the de facto fuel for the low compression engines overnight ( I don't recall seeing any STC's for that, and to this day, wonder about the showning of design compliance for 100LL!). Yes, it has half the lead, but is still very troubling for those engines, and causes maintenance difficulties (and forced landings) which did not occur with 80/87 (lead fouled plugs). No flight manual supplement was issued for low compression engined aircraft which were now expected to use 100LL, to explain how to operate the engine to achieve the best relibility.
So, yes, Mogas is a very different fuel to Avgas. But testing has been down in amazing amounts, and Mogas which meets the prevailing American and Canadian (I cannot speak for other nations) has been shown to meet
all of the resign requirements for the STC approved aircraft and engines (with some limitations, in some cases, as noted). 30 years of excellent service history are very reassuring too!