In my experience, letting band members control their own monitor mix during a gig via phone or tablet is asking for problems. What most people do is turn channels up to hear them better, they don't think about turning other channels down. Invariably, the monitor mix gets louder & louder and you get into feedback territory. When the wedge starts to squeel they'll look at whoever owns the PA and give the 'it wasn't me, I didn't do anything' look. Monitor mixes should really be sorted out in sound check, with only some minor adjustments made as the gig starts. I would suggest that unless you really trust musicians to be sensible with adjusting monitor mixes, then only one person in the band is responsible for monitors & mixes to keep things under control.
Of course, if they're using in-ears, let them loose on their own mix every time as it doesn't have any bearing on stage volume.