dp - as a VFR user you will only ever use it as a supplementary aid to navigation, so there is no training requirement (you don't receive radio nav.aid training for a PPL, right?). As mentioned, there are harmonics of the GPS frequncies that may cause interference - result is your GPS will not work, so no problem, you can't use it. Major handheld problem is probably"fat finger syndrome" - putting in the wrong information, or reading the wrong information. Keep your eyes on the outside!
IFR use is slightly more problematic as there is the navigation integrity requirement to deal with, which requires more satellites in view than for a 2D navigation solution alone. Coupled with the receiver mask angle (that eliminates low horizon satellites from the navigation solution), this can create an availability issue. Integrity monitoring prediction services can assist in the operational planning aspect of this.