I have the Parrot Flower Power unit and it works reliably enough. I take all the information as relative and not exact (except temperature which is typically spot on).
I'm using it to monitor a single pepper plant currently bearing about 11 peppers and I use android devices to interface with it.
PFP monitors 4 things:
Light - Not all that useful of a metric as it is shaded by the plant. I use a lux meter to get better results so I know what's what at the canopy.
Water - Useful and within reason. You need to put the monitor close enough to where you water the plant for it to be useful. I am typically seeing 10-25% on their scale though it's hard to tell what that means or what is actually enough based on that. You could just trust their notification system but you'll get a lot of false positives. It does seem accurate as I keep the soil lightly saturated pretty much all the time.
Fertilizer - Hard to verify the usefulness as I haven't been using it long enough to actually need fertilizer. It's not telling me I need any yet so that is a good sign.
Temperature - Useful and accurate by all accounts.
All in all the highest benefit is seeing the day, week, month, and year charts along with the live data feed which accurately depicts reality for lighting and temperature.
Mine is inside so I cannot attest to reliability outside and it will need to be in range of the bluetooth monitor. I don't think it is intended for outside use but it should be waterproof and should work just fine.
It you are planning to use it for a fruit tree, you ought to be able to see when you water it, or when it rains heavily enough but the fertilizer detection will probably not be of that much use. Temperature and lighting will likely be the most useful metrics for you.