Maybe I, being quite old, can shed some light here.
Teacher assessments: I am very supportive of teachers, however, there is no doubt about it, despite our best hopes and desires, not all teachers 'are equal'- just like the rest of us! And with the best will in the world, IF a teacher's on-going career advancement, or school's OFSTED (and LT position) is dependent on things like how well the DC they taught apparently perform, wouldn't anybody be persuaded that some massaging might be the best policy? And IF you had the opportunity to push a couple of kids 'over the line' who you knew actually weren't 'that good' via 'teacher's assessment' rather than National, independent SATS , might you be tempted?!
My DB went to a SM, 1972-77.
His reports pretty much glowed, Y7-10, then, suddenly! Report/parents eve:- DB's performance has dropped away dramatically this year! What can have gone wrong? DB won't do well in his CSEs with this performance, you know! Which was true- 2 low grade CSEs. Naughty DB! Til mum and dad, a year or two later, compared notes with another parent whose DC had just left- turns out every DC that this other parent knew (the game was approaching being 'up' by then) at that school followed the same trajectory- apparently stellar Y7-10, then..What went wrong?! Cue lots of hand wringing from the school but- 'What can you do?'
So, no, actually, to 'teacher assessments'.
Q: 'As far as I remember degrees don't just rely on exams and as far as I have heard, GCSE's include an awful lot of course-work - that's the equivalent of different ways of testing....'
You need to read the Press. Gove's huge on removing course-work from GCSEs- why? Employers and FE colleges are bearing the brunt of the fact the DC don't know the facts as they've not been independently assessed.
Q: 'There seems to be some underlying hostility here and just curious to know what it's about....?' Hostility to what? For or against SATS?
Finally poor SATS results confirm poor teaching. They maybe don't improve that cohort's 'bad teaching' but they sure reveal it!