Curt Aldredge has concluded there’s no proof hollow icons are harder to interpret/recognize than solid icons:
Johnson’s warning against using hollow icons in user interfaces just isn’t supported by evidence from real users. For one thing, an icon’s style doesn’t exist in isolation, but interacts with other attributes like color to create compounding effects on usability. Furthermore, less than half of the icons in my set of 20 performed better in a solid style than a hollow style. A different set of icons would likely result in a different overall result.
Me? I don’t care if hollow icons are harder or easier to decode than solid icons.
I’ll repeat myself 50 million more times if I have to: hollow icons are not icons, they’re wireframes of icons.
Different strokes for different folks, but I think a more appropriate approach to icon design is thinning them out, not hollowing them out. Let your icons stay solid, just streamline them so there’s less area to fill in.