In my experience, it's every bit as bad as you imagine:
When I was an undergrad, for whatever reason Univers was the default typeface used for computer document production (much of which was done using the Scribe markup language), and as a result, tons of lengthy documents were printed in Univers, including a lot of stuff that was book-length. [Why it was Univers I'm not sure. I don't think they had very many fonts though.]
It was absolutely dreadful: reading page after page of text set in Univers was downright exhausting.
I think Univers is not such a bad typeface in the abstract, but because of the way it was used, I came to loathe it (and right now looking at some Univers examples on the web, I still feel a twinge of disgust...!).
After I had been there a few years, other document production systems (such as TeX), and newer printing technology (like the Laserwriter I with PostScript!) came into use, and the number of supported fonts increased. Many people shifted to serif fonts for their documents, which was a huge relief...
Oh man, my first university used Univers (and not just Univers, the condensed version) for all their signage and handouts as well.
I almost feel like with a font like Univers the similarities between letters could saturate out your pattern recognition quite easily (imagine looking at these same stems and bowls all day long), and that perhaps serifs (or at very least, differing lettershapes) add a much-needed bit of stochastic resonance to the whole process. Maybe the uniformity of modernist typefaces isn't all it cracked up to be. </idle speculation>
