No shit, Sherlock. Of course, all of this was obvious to any of us with more than 5 minutes of industry experience, alas this field seems super prone to reinventing the same things over and over again, making the same mistakes and never learning from the past.
Also, this article does not even mention the fact that SPAs are terrible for SEO. Of course, that's not an issue for YouTube, but it might be a huge issue for your clients, unless it's ok to not have any search traffic.
It is definitely not solved for the vast majority of SPAs out there. Where it is solved, it is not solved for free or cheaply, as it would've been if these apps used server-rendering tech.
Kind of, but it is done separately from regular indexing, at a much lower volume and it is filled with loads of gotchas [1] that can make Googlebot skip your site.
In practice, what happens is Google spends their JS rendering resources on the top sites and will very rarely render a new site. Generally, assuming you stay clear of all the gotchas, Googlebot can eventually "see" your JS-rendered content but it will certainly take a lot longer to be indexed and, therefore, it will take longer to appear in search results. If you or your clients rely on organic search traffic for your site, then picking client rendering based tech is certainly a bad decision.
Also, this article does not even mention the fact that SPAs are terrible for SEO. Of course, that's not an issue for YouTube, but it might be a huge issue for your clients, unless it's ok to not have any search traffic.