Tax reform needs to incentivise economic productivity to finance the other functions of government, which in turn also need to enhance productivity (eg through education and infrastructure) while also meeting other non-productive social needs (eg defence and welfare).

But one aspect in that equation that I think is often ignored is the responsibility with which government expenditure is deployed as this can entail conflicts of interest and fraudulent payments to related parties.

So the good aims of tax reform can be negated by the hidden cost leakages of corruption. This is not only a problem in developing countries.

The topic is a complex one and related to the political economy rather than state finance or macroeconomics. Relying on the developing or underdeveloped countries' experience, I would recall the "domestic political will" as a key determinant to the success of tax reforms. I clearly remember a quote from the president of Georgia, Mikheil Saakashvili: "the most difficult thing to do was to force the citizens to pay the taxes". Tax is a concept that is usually associated with political and institutional endowments of a country. The rulers of the country might not be interested in the collection of taxes for several reasons: maybe they made a deal with certain shadow interest groups that are involved in big "games"; maybe there is a huge revenue stream into the country and nobody is interested in the alternative revenue sources, simply their marginal utility is low. The measurement of the success of tax reforms is simple. If the tax revenues of the state budget are increasing and the government expenditures towards the social needs are in an increasing trend, then we might have an initial stepping stone to argue about the certain measurement of it.

Tax reform is successful when tax collection increases, budget revenues increase, and on the other hand, expenditures on education, healthcare, social activities, etc. increase.