I use "L" all the time, slippery conditions or not. We've just gone through a snowfall where over the course of a week or so my back lane started with crisp, fresh snow and then degraded to compacted snow and then ice. Every day I depart down the lane I test my traction because it's always worst-case compared to the city streets. I've never had a problem with "L" mode.
Yes, if you take your foot completely off the accelerator pedal the wheels can slide, but the car modulates the amount of regen and the car still comes to a controlled stop. But you shouldn't be taking your foot completely off the pedal in those kinds of conditions! In "L" mode removing your foot from the pedal is the same thing as applying too much braking force, and you'll slide if you do that too!
A lot of people seem to have this "all or nothing" attitude to "L" mode, but you can modulate the amount of regen in "L" just like you modulate the amount of braking force using "D" and the brake pedal. There's no reason to slam the brakes on every time you hit the brake pedal, and there's no reason to just pop your foot right off the accelerator every time you slow down in "L" mode.
The only potential benefit I can see of "D" mode is that use of the brake pedal can engage the rear brakes as well as the front ones. But here's the thing - light application of the brake pedal in "D" mode engages regen first anyway - so you still slow down using only the front wheels. By the time you've gotten to the point where you're using the actual hydraulic brakes in low traction conditions you're probably already sliding anyway, and shouldn't be pressing that hard. And if it's an emergency stop then just stomp on the brakes and let the anti-skid braking do it's work - that works the same in "L" or "D" modes. But if you find yourself doing emergency stops more than once in a blue moon then it means you're in the habit driving too fast or too close for conditions.