Ford had a shot when they bought Stewart Racing and changed it to Jaguar F1. They made a complete cockup of it as they tried to make decisions from a corporate level, which will never work.
Andretti has a great pedigree and Cadillac (actually GM) has made great showings at Le Mans over the years so I don't see an issue from the vehicle supply side. Who makes the engine will be a factor and make or break things right off the top.
Merc and Ferarri are putting up a squawk as they are worried the new team will be competitive right out of the box and they may have to share the toys.
GM continues to have dealings with Ilmor, who make the Chervolet IndyCar engine. Ilmor used to make the Mercedes F1 engines that McLaren won championships with. Ilmor was split into two companies, one half is now part of Mercedes, making the current power unit for Hamilton, Russel et al., and the other half, still called Ilmor, working with GM.
Even if Cadillac won't be creating an engine for the current formula, they have the facilities and expertise to ramp up to the 2026 formula, just like Audi is planning to. It certainly would be easier than Red Bull setting up their own Power Trains factory.
(With that said, while Cosworth historically is associated with Ford, and sat out the current costly engine generation, they've worked with GM in the past as well, so that's another existing facility that could make F1 engines for the Andretti (and other) entries.)