Virtually any geo-engineering solution will have horrific side effects. It's a matter of how systems work. You can easily control a linear system with a feedback loop. We know all of the math and the equations are easy (if sometimes tedious) to solve by hand or by computer. For some non-linear systems, you can approximate them as linear and control them that way. For other non-linear systems like the climate, which are chaotically non-linear, only god can help you.

Hell, even if the climate did respond linearly (enough), controlling it would still be very difficult. For an N-order linear system (that is, there are N number of states, or energy storing "devices" in the system), you need to continuously provide N inputs in a precisely calculated way to control it. You also need at least N sensors to be able to keep track of the N states, whether directly or indirectly. On top of that, we would still need massive amounts of energy and materials to actually provide the control inputs.

If you did all that, then geo-engineers could control the climate in a predictable way. But the thing is, in political consciousness, very few people actually know what it would take to control the climate. Most people think of geo-engineering as a cheap and quick way of solving climate change. It is not.

To use an analogy, imagine you are driving your car off a cliff. You could press the brakes (reduce emissions), or you could build a paraglider, attach it to your car and then fly off the cliff (geo-engineering).