Limited compatibility means the goggles have an unusual shape or strap design that doesn't play nicely with many helmets. You might end up with a "gaper gap"—that cold strip of exposed forehead between your goggles and helmet—which isn't just uncomfortable, it can also cause fogging. The strap may slip, and you could feel awkward pressure points on your face.
These goggles tend to be fashion-focused or built for very specific setups. If you happen to have a helmet that matches their unusual shape, they could work fine. But for most riders mixing and matching brands, they're risky.
If you're set on them, try them on with your exact helmet before buying. If there's any gap or the strap won't stay put, keep shopping—cold foreheads and fogged lenses will ruin your day on the mountain faster than you'd think.