If the power fails they'll come on at the wrong time.

Of course the pumps will not be able to run when there is no power but it might be less than ideal to run the pumps at the wrong time.

That said, you could solve that fairly easily by running the timer circuitry off a rechargeable battery and drive the pumps through a relay. When there's power the battery recharges from the grid, when the power has failed for a while the timer hasn't lost track of the time of day.

I wonder how much an SMS message costs an indian farmer (it also requires an extra cell phone), it might very well be a substantial part of the cost of operation. In the end it's a convenience thing (you don't have to walk) which is worth some $ or it would have to save money or create a better yield.