Once you begin (without actually reducing co2 emissions), you can never stop.
You will have to increase the rate of SAI every decade, in perpetuity.
If, for any reason (war, political instability, economic burden etc), the SAI stops the rapid change in temperatures will be devastating to ecosystems on a level that would make our current rate of change look like paradise.
From what I remember of the effect the Mt Pinatubo eruption in 1991 had of global warming the effects disappear within 5 years. If humans want to reverse warming we will have to push well past the "safe max co2" of 350ppm.
My little End of SAI Nightmare Senario.
Its 2100 After 50 years of SAI pegged as if there is only 200ppm of co2 in the atmosphere. The actual level is some where between 800 - 1000 ppm.
The SAI stops. In effect the earth goes from 200ppm to 800 - 1000 ppm in five years.
Climate engineering solutions have the problem of being inherently international. One country can't just start injecting aerosols into the stratosphere without every other country on Earth freaking out. It'd require a level of international cooperation that just doesn't exist.
I'm sure that it would cause international tensions and lots of saber rattling. Maybe it'd amount to nothing, but if China started doing this in the next few years you know the US would freak the fuck out.