I've had my Nexus 5 for about 2 months and noticed several major modem issues with it.
The first is wifi randomly just not working. It will supposedly remain connected but I wont be able to load anything at all. I can turn wifi on and off and still nothing. I have to actually shut down the phone to get it to function again.
Other times when the WIFI will work fine. If I ever switch it off and go to my cell signal it will not go on to data or lag to go on to data or go on to 3g data when I'm in a zone with LTE coverage. Often I have to restart the phone to get it to pick up data or LTE as that is faster than actually waiting it to switch. Sometimes It just doesn't switch.