Yes, my experience is firsthand via DD for all of the above. The one exception is that I have NOT seen a
lot of assessments under FLVS, so I can't say if those are in great alignment or well-written generally speaking. What I have seen has been not awful, but not awesome, either.
Understand that many students
don't have major issues with any of them-- but that gifted students (IME/IMO) almost certainly WILL. MOST students in HS science classes are memorizers, not "understanders." Therefore, they aren't really engaging the way sciencey kids do, and almost certainly not the way HG+ ones do.
There is at least one other member here that I
know would agree with me on all of the above points. I think that he ultimately withdrew his PG child partially on the basis of determining that the underlying problems were structurally-based in curriculum design, and support for the same.
Those courses are written by contract-- low bidding, basically-- and often in virtual sweatshops, which is
precisely the problem with Virtual Sage. I know because I tracked down message board posts from tech writers on contract who warned about them on jobs boards-- that they paid poorly and not on time, that their timelines were INSANE for product delivery... and that no, it didn't really require subject expertise, since they provided the electronic version of the course texts... (no worries, right?) or that in courses without texts, not to worry because the course syllabus was quite detailed and gave a lot of guidance on the types of content to include. Oh, and that the assessments weren't part of the 'package' either way-- those were contracted separately.
All of that information explained a VERY great deal about the problems with the curriculum.
One of the questions from a Virtual Sage course was
so bad that I remember it even all these years later--
in this question, students were expected to "understand" that seismicity and vulcanism are
unrelated phenomena. 
Well, my daughter, being a fairly science-y kid, and living ON the ring of fire, 'understood' no such thing. And argued vociferously on this point, in fact. All she did was alienate the teacher, who labeled her a trouble-maker.
Anyway. That kind of thing is stunningly common in online courses-- the problem isn't any ONE thing, it's everything. Bad curriculum writing (at best, mediocre, IMO), AWOL teaching, and assessment writing by people who clearly should have had more rigorous gen-ed coursework in college. Fix any ONE of them, and it becomes more tolerable. Fix them all, and the experience can be quite a good one. My DD's AP Phsyics class was about a 1.5 out of three proposition... teacher was great, and the assessments and curriculum bit were about two stars out of four each.
It just usually isn't that good-- for the reasons that DeeDee noted. It takes a LOT of time to teach well, and short-cuts and canned curriculum to "automate" the process don't really work as an adequate stand-in most of the time.
I mean, yes-- I'd choose this over actively AWFUL B&M science, probably. Sure. Well, some of the online stuff is pretty awful, too, honestly...
