We will likely need to find a way to lessen the effects of these accumulated particles, similar to how we are currently needing to lessen sonic booms resulting from going supersonic. Otherwise, the dangers would really limit where you could come out of warp.
Not really, because most of space is empty, well, space. That's why it's called that. Fiction portrays it as being cluttered on a scale that reflects our Earthbound experiences, but that's absolute malarkey. In reality, it's immensely empty beyond our ability to comprehend. The only way you're likely to hit anything in space is if you're deliberately aiming at it. If you come out of warp inside a populated star system, then, okay, there is a small but nonzero chance of hitting an inhabited planet, space station, or something, but then the best option is to come out of warp at an angle to it, or pointing away from the orbital plane where most of its objects would be found. So it's a manageable risk, as much as any collision risk in space is manageable.