Hi Scots_cleaner.
Personally I am a fan of back vacs, but on sites where I have recommended their use it seems that many cleaners don't like to use them.
The usual negatives include them being uncomfortable, causing back-ache, and becoming very hot on the back as the motor heats up. Also to reduce their weight, the refuse capacity is much smaller, so what you gain on productivity you lose on having to empty/change the bag.
Nilfisk do a great back vac but from what I have heard the cleaners do not like to use them.
Similarly it seems they are against uprights as well.
What I would say, with particular relevance to domestic cleaning, is that productivity increases are best seen in large open plan offices. Not many homes resemble those!
Musicman