This depends a lot. Many pedals will never actually hit 100% wet (kill dry), which is an invitation to get phase cancellation in a parallel loop. Also, any EQ, noise gate or compressor needs serial.
Actually, it´s pretty weird that amps even have parallel loops because I think most guys don´t know how to use them properly. You can´t have any dry signal coming back to the return jack, which makes planning for something like using several pedals in a row a potential nightmare.
With that said, a proper parallel loop (like any split and mix system) does wonders for clarity of tone if you use time-based effects.