A current trend is for many websites and mobile apps to use an outlined icon style.
Do visually impaired people find it hard to recognize these icons?
See for example this screenshot from iOS.
Icons are all about symbolism.
Line icons are good in providing visual references globally. They are adapted for high DPI screens as well.
To make the iconography more accessible some standard practices can be used-
Avoid details: that goes generally with any icon set being designed, but especially if you are planning to provide high accessibility.
Contrast Colors: Just like any other UI element, revert colors or contrast colors can be provided to enhance accessibility.
Sizing: thin line icons can cause trouble, especially with small strokes in a smaller size. iOS handles it by using a zoomed in mode which scales all the elements on the screen.
I really refer to this article when looking for accessibility issues with icons and typography -
Also, one of the most famous icon set for the web - Font Awesome added accessibility support which is standard and works very well -
Not everyone with visual impairments are totally blind, even if they meet the criteria to be officially registered as blind, e.g. people with macular degeneration can still see (albeit with tunnel vision).
So as long as the iconography supports a text equivalent, you will always support people who need additional support to help understand the purpose of the iconography.
If you are developing native mobile apps you must leverage the accessibility features of the mobile OS platform, but if you are creating a responsive website (e.g. html 5), you must include a text equivalent, either by creating them yourself or by leveraging the great work by companies like fontawesome (for font-based icons, aka font glyphs) or whatever accessibility features are included in your preferred UI library.