The most common complaint in this regard is that the rules placed on people who are on probation are arbitrary and unfair or that probation officers treat those on probation unfairly or arbitrarily. While it is always important to work towards a fairer and less arbitrary system, the system is not fatally flawed in this way.
Of course, there are times when the decisions about who does or does not get probation will look arbitrary. There will be times when the actions of a probation officer seem arbitrary. But that is unavoidable in any system that relies on human beings to judge other human beings.
On the whole, probation is surely preferable to being in prison. Therefore, it seems that we should not get rid of the system. Instead, we should try to improve it where possible.