Major points are covered in Danielillo's answer. My opinion is similar but I have some notes and I'll make comparisons with some sans-serif fonts.
So in general there is nothing bad about "Corbel" - it's just not the most readable font out there, thus maybe not the right choice for a long text.
Apart from being a sans-serif, following features play role here IMO.
Letter spacing and proportion
"Corbel" has loose spacing and expanded proportions compared to some more
readable fonts. Here is comparison with "Liberation Sans" font which has dense spacing and narrower glyphs.
Corbel / Liberation Sans
So generally speaking, loose spacing is bad for readability.
Note: more spacing and wider proportions are not always bad. Namely a font with such features can be better readable at very small sizes.
Consider downsampled example (same fonts as in the above sample):
The first structure remained slightly more comprehensible due to wider proportions and spacing, whereas the words in the Liberation sample became optically smudged.
I've noticed that Danielillo used the term "modulation" in his answer, though I was not aware of such term. I'm not sure if there are precise terms for such things.
So in general, fonts which have oversimplified strokes, i.e. strictly aligned and all of the same width, are considered bad for readability. It is hard to explain this effect with words, so simply put - some stroke 'play' and even some inaccuracy in stroke alignment helps to 'relax' the overall image structure and thus can reduce the eye-strain.
As an evidence example - I always remember the Comic Sans MS font. Despite it's childish design, many people find somewhat pleasing in it and this relates to the mentioned positive effect caused by such inaccuracies in strokes.
And of course variable stroke width is just a natural part of glyph morphology, e.g. stroke transitions and connection areas require careful manipulations with strokes widths.
Compare with a very similar font which has slight stroke width 'play':
Corbel / Candara
Well, IMO such digits are annoying.
This note kind of contradicts with my 'play' theory, but it is just too much play here.