The general consensus is that phone apps for shot timing aren't good. They don't seem to be super reliable in terms of catching every shot and not considering non-shots to be events. Worse, IMO, touch screens are a bad combo with dirty hands and no-look operations. Most people's phones don't have big clips on them.
I occasionally use one for dry-fire in my basement simply because I already have the phone in my pocket and don't have to switch modes on my real shot timer. But even for that, if I'm doing a "real" dryfire session, it's worth digging the real shot timer out of my bag and changing the start setting and putting on the par time, just because a real shot timer is better than a phone app even when it comes to just generating par time tones after a delayed random start.
Also, since this thread is now a little old, I would add that I have been very impressed by a friend's AMG Lab commander shot timer. They're about 50% pricier than my go-to pocket pro, but next time I buy a shot timer, I'm pretty sure it will be one of the AMG things. Bluetoothing all the times (all the splits for every shot in a stage) to the practiscore tablet is an amazing feature.