The beauty of this setting is that it falls back 100% gracefully. Basically if your OS and/or browser doesn't support it, then you just see the "normal" version of the site.
Because Dark Mode is only available on newest versions of most operating systems, it is also reasonable to assume that the people running a dark mode compatible device would have a modern browser.
So all-in-all, I think compatibility is a moot issue.
One of the keys to B2B sales is considering the economics of risk.
From a business standpoint, I'd be concerned about taking a dependency on a web service with the business model I see here. Free for open-source means it will be harder for the company to make money and the harder it is for the company to make money the more likely it is to disappear. If the company disappears, a business taking a dependency on the service is in worse shape than if it bets on a slightly more difficult to implement alternative.
In addition, a large number of non-paying users creates a support load that requires resources that could be used to develop a paying customer base. This also makes it somewhat less likely that the business will be sustainable.
It is a valid point and there are chances of that happening. As of now, I do not expect a huge load from open source projects, but only time can confirm that.
I guess it depends what your definition of "serious" is. This doesn't sound like it would ever affect me.
As far as I can tell, it will only cause problems if you don't delete entries from other tables that have a foreign key pointing to that row. There may be some cases where you need to do that, but generally isn't it better to properly delete all the rows from other tables that use that foreign key when you delete the primary key?
