There are some important fundamental differences between Shim/ST indexing systems that make successful parts mixing more than simple cable pull and cog spacing, critical as that is. There is also a difference between what may work for us with random components and a bike in our stand today and what happened in a production environment 30yrs ago.

Shimano indexing precisely centers the guide pulley under the cog with every shift, in both directions. The cog tooth profile helps grab the chain before the guide pulley is centered, and the floating guide pulley both keeps the drivetrain quiet and helps compensate for out-of-spec tolerances throughout the drivetrain. Shifting is smooth, fast and quiet, and the system is fairly tolerant of incorrect cable/casing, less-than-ideal chains, long rear drop dimensions, etc. Dealer setup is fast and easy, and the bikes don't come back frequently needing adjustment.

Suntour couldn't use Shimano's cog tooth profiles or floating guide pulley without infringing patents, and their indexed components were rushed to market in desperation after a disastrous 1984/1985 sales season. That was the year of "Doesn't click, doesn't sell." Dealers couldn't get enough road bikes with Shimano 600 SIS, and thousands of road bikes with Suntour Sprint sat unsold.

Suntour indexed downshifts relied on a little bit of overshift built into the lever to make sure the cog has the best chance of picking up the chain, with the guide pulley hopefully settling into a centered position after the lever is released. Upshifts depend on precise guide pulley/cog alignment and the inertia of faster chain travel onto smaller cogs. The system was very intolerant of mis-spec'd components and frame dimensions, it generally shifted slower, had a narrower range of adjustment, and took much longer for dealers to setup. Customers came back for adjustments far more often. Accushift was very soundly rejected in the marketplace, and as Suntour lost spec they died.

There are additional wrinkles. I'm not sure what the CTC archives say, but Suntour R&D acknowledged three distinct rder shifting geometries, which they tried to cover with one lever spec. They also initially shipped tens of thousands of out-of-tolerance freewheels. US dealers were shipped go/no-go gauges to check cog spacing, and were asked to insert microshims between cogs to correct spacing. Suntour dealer support had to ship dealers improved chains and freewheels for new unsold and recently-sold bikes, and Suntour had to send teams to OE warehouses to rework bikes before they shipped to dealers. It didn't take long for dealers to demand product managers only spec Shimano.

Having lived through all of this I can never find any personal enthusiasm for Shim/ST mixing. I'll never discourage anyone from trying, have fun!, but consider it a gift if it works, and maybe don't assume if the math seems to work the parts always will. Shimano got it right but Suntour never did, and there's more real-world reasons for it not to work than to work.