Hi Francios,
I don't know which Navigation software you are using; I use EasyVFR in conjunction with a cellular iPad.
I have the GPS dongle on my adsb only PAW, as recommended (still awaiting delivery of the new Bridge plus licence so I will gain full functionality using a Pi2B), but generally use the internal iPad GPS for actual navigation. Never had any problems with it, including a flight last year from Shobdon to Alderney. Granted this trip predated slightly, my involvement with PAW.
EasyVFR gives you the choice which GPS source to use within its Flarm interface settings. I normally use the iPad GPS for flight planning and then just go flying, without sodding about having to changeover settings to use the PAW dongle. Having the dongle allows for rapid trouble free boot-up of PAW and provides visual confirmation that all is well when the dongle green led starts flashing. It also gives you a measure of redundancy if one of the GPS's plays up.
Lee's comments regarding the iPad GPS are correct. It is "assisted" GPS meaning that the Wifi and cellular triangulation etc allows for more rapid position fixing, particularly when indoors, but when under a clear sky the GPS engine functions as well as any other "tablet" or car navigation source that I have ever used.