Read through the forums, and you will find a lot of info about the signal indicator for Nexus 5. The phone in general I have found gets decent signal on par with other phones that are available.
The issue as I have come to understand lies with the Nexus 5 running vanilla Android and not adhering to any particular carrier's algorithms to indicate signal strength .
As such the phone often shows one to two bars lower than it should and sometimes reads no signal when in fact the phone does have good signal in terms of dBm.