Good graphic design practice is san serif fonts are generally better for the web and serif fonts are better for print. Both these relate to long format pieces, with paragraphs. As joannekwan said above, san serif fonts when written in long form can be straining on the eye in print form, in the same way serif fonts can strain the eye when reading long form on a screen.
Comics though, within the word bubble format, I would say doesn't fit with these guidelines. That's not usually more than a few lines of text in a bubble, and the imagery the viewer looks at in-between reading allows the eyes to rest.
The people who may be complaining are complaining about san serif fonts more in a general sense. San serif fonts are very popular right now in the graphic design world, they fit in with the minimalist trend that's taken over the field in the past couple year. It could just be people who are tired of the trend and want to see something different. I personally, as a professional graphic designer, see no problem with san serif fonts. Use the font that fits the job.
When it comes to comics though, I think complaining about a style of font is irrelevant, as the font should fit with the tone of your comic. If the font works with your comic, people shouldn't even notice it.