Physically separated bike lanes are even worse. You can't get out of them easily if you come across a lot of debris. And they are harder to street clean, so debris accumulates more.
As a counterpoint to the "harder to clean" argument, I can offer the following.
I've just been to Montreal, and with temperatures well below freezing and lots of recent snowfall, the segregated bike lanes were completely clear of snow. Every day. In the places I've been around town, I am not sure I've ever come across any significant debris, in any season.
This is in stark contrast with many (especially older) bike lanes in the UK, which are basically unrideable in any season as they're covered in drain covers, potholes and debris.
I think the difference here is not so much the difficultly of cleaning as the commitment to doing it.
Why would they be any harder to clean? A city can just have one of their sweepers be a much smaller, cheaper, sweeper. Likewise for snow clearing. A much smaller and cheaper machine can often be used.
