How bad is geo-engineering?
The bad part about geo-engineering are the unknown unknowns, to paraphrase a certain US politician.

Our climate models are wrong.  All models are wrong, but some are useful..  Our models are useful, but not quite useful enough to trust them when they tell us massively spraying stuff into the stratosphere or the oceans is mostly harmless.  Our models can't properly reproduce our current climate.  They agree about some trends, but there's lots of stuff that they're missing.

We used to use lots of toxic chemicals in our fridges.  That was a bad idea, so we found some stuff that wasn't toxic.  Great, no?  Decades later, scientists realised this was destroying ozone above Antarctica.  At first the measurements were rejected because they didn't make sense.  Contrasted with our understanding of the atmosphere, and why would ozone suddenly disappear anyway?

Our climate system is complex.  Very complex.  Mess with one of the symptoms, and who knows what side effects are going to be?  It's a risk that we cannot quantify, and those are probably the scariest risks out there.  If even with stuff that doesn't seem remotely connected to each other (fridge coolants and ozone) we almost messed up very seriously, who is to say actively messing with our atmosphere or ocean isn't going to have nasty unintended consequences?

Better than offsetting one set of geoengineering (deforestation, ocean acidification, enhanced GHG emissions) with another equally radical one, it might be safer — from a risk management point of view — to try to not do any geoengineering at all.  Certainly as long as our understanding of climate is just starting.

To borrow an analogy from medicine: preventing is better than curing.  Patients are sometimes prescribed medicine where the side-effects are so bad that they're almost lethal by themselves, but only when the patient is surely terminally ill, so there is "nothing to lose".  We do have a lot to lose on Earth.

Note that weather models are initial value problems and climate models solve boundary value problems, so mathematically speaking, the two are completely different.