There is an elaborate thread about this (I'd guess) issue, an the conclusions are that the library would need some updating, but since there are other libraries already, just use them meanwhile.
Default example for Asset Tracker GPS prints erratic NMEA sentences to serial
Some of the side issues and possible solutions regarding serial buffers and such are discussed in the background already.
I'd say the bad results are not due to poor hardware but rather on the software side. Although this might sound like an easy cop-out the library was in big parts just taken from the Arduino library and adapted for Particle. But I'd guess Particle will do some improvements (or replace that lib with the already working one).