I heard part of the Chromium choice was that Edge developers felt they were already spending too much time in the Chromium codebase to support Windows initiatives even before the decision was made. For instance, Microsoft wanted to make sure all the browsers to support ARM64 on Windows natively, Mozilla did the work themselves (and it was basically already in their build farm when Microsoft asked), and Google asked Microsoft to PR it (which they did).

Not sure if this is accurate but when put like that it sounds like the worst possible reason.

It's not a great reason, but it is an interesting failure [1] case of the open source model, when someone else's open source project becomes your own liability because so many of your own customers use it.

[1] Arguably a success case in some viewpoints; it's sometimes why people promote open source.

Beats having to maintain bug compatibility to keep popular proprietary code running though

Though at least when Microsoft was doing that they were documenting those bugs and bringing them all to web platform standards boards for review. Now they are more likely to contribute such bugs to the web platform again without documenting them, leaving probably poor Mozilla the only ones left to notice.

Tech doesn’t exist in a vacuum. When team W is tasked with X, and has the possibility to use tech Y, which they are seriously acquainted with, and tech Z, which they barely touched, it makes perfect sense to go with tech Y as it’s much more efficient to get to speed and evaluate risks. In fact, as long as risks are evaluated and cleared regarding Y, there is no reason not to go with Y.

As for actual risks, remember, this is MS, not some random shop: at any point if they’re dissatisfied with Chromium in any way, they have enough firepower to fork Blink, just like Google forked WebKit from Apple, and like Apple forked WebKit from KHTML, and do whatever they want.