The NPPL is essentially the old UK Microlight PPL extended to light aeroplanes - and as such the test standards are the same, and pretty high: just a slightly thinner syllabus and lower medical standard.
So far as I know, the standards of microlight pilots pre-NPPL was pretty much as high as the PPL(A) as was, and the standards now are indistinguishable.
So, in my opinion, an NPPL(SEP) holder is on average neither more, nor less, safe than a JAR PPL(SEP) - they can just do a little less with their licence.
G