Personally, unless there's a very obvious narrator, or near-narrator, I haven't the faintest idea how to tell the difference between omniscient when it's apparently OK to change from POV to POV within a scene without a break, and head-hopping when it isn't. And I sometimes get the impression from comments that which it is depends on whether that particular commenter liked the book or not, so it's omniscient and therefore acceptable if the book is liked, and egregious head-hopping otherwise...
Anyhow, I got a free copy of Heirs of the Blade last year, which is the seventh book in The Shadows of the Apt series and first published in 2011. As far as I recall, it had no head-hopping in it at all. (Bear with me.)
Since I enjoyed it, I went and bought book one, Empire in Black and Gold, first published in 2008. There, the POV jumps all over the place, within scenes and from paragraph to paragraph. (Or it may be it was written in omniscient and I'm too thick to see the difference.
)
So, head-hopping wasn't enough to prevent an agent taking Tchaikovsky on and a publisher buying his series, but somewhere along the line he has changed his writing habits. I've no idea why, but if it came from his editor, why didn't he/she ask for the first book to be "corrected" before it came out?
As I've said before, I think this is something we agonise over more than does the general public -- the POV issue certainly didn't stop him selling a lot of books, and getting readers for his second and subsequent novels. On the other hand, he has stopped doing it, so presumably someone has raised it as an issue.
As to immersion, it never stopped me being immersed in a story before I was told (here) it was wrong. So, again, I wonder if that is simply an ex post facto rationalisation, rather than a reason.