Ubuntu has some quirky elements, such as the corners created where shoulders meet stems on the letters a, r, n, m, h, p, q, and u. The quirks don’t undermine the overall texture, rhythm, or readability of the font, but it does give text a slightly “futuristic” feel which may or may not be appropriate for a project.
Even with the “futuristic” feeling, I categorize Ubuntu as a humanist sans because it has generous apertures, bowls with implied stress (on the b, d, q, p), a curved foot on the lowercase l, and a relatively humanist italic.
Ubuntu’s x-height is slightly smaller than Verdana, yet holds up very nicely at smaller sizes due to generous apertures and bowls. The bold weight is a bit heavy for my taste, though it is still legible on screen. While a semi-bold is available, it is not heavy enough to create a good contrast to the regular weight.