Hey Bajo,
I'd love it if you could prove me wrong on this, but the problems I see with the Monotributo include:
1. Its classification system is precisely not income based, so it is regressive at both its upper and lower ends, with lower income monotributistas paying a higher share of their income and higher income monotributistas paying a lower share of theirs.
2. It encourages work en negro. On the lower end of the income scale, many, many people I know cannot seem to get jobs en blanco (situaciones de dependencia). Their only option in terms of getting some of the benefits afforded employees is to pay for a low category monotributo. This way they can at least have some kind of retirement, health insurance, etc. So in effect, the monotributo serves as a stop-gap measure: it keeps poorer people from accessing the greater benefits of working en blanco and lets the public administrations off the hook about having to get everybody properly registered.
3. Continuing with the previous issue, lower income monotributistas are at an extreme disadvantage in terms of obtaining loans or participating in many government subsidised loan programmes. We saw this recently in Jujuy province, where the state has a huge mortgage programme to address the recent land issue. The assumption there was, if you're a monotributista, you're rich so why do you need a state subsidised mortgage? This reasoning is excellent, but the problem is that monotributista does not always equal rich landowner. This is in addition to being treated as second-class citizens in things like obra sociales, etc.
I haven't really thought much on a better solution; all I'm doing here is enumerating some of the more egregious problems I've witnessed. And like I said, I'd be thrilled if you could show me where I'm wrong on this.
Best,
Ed