What are the risks of developing geoengineering? Wikipedia page for geoengineering entry list a lot of proposals to decrease global temperature:

Solar radiation management methods[5] may include:

Surface-based: for example, surface-based mirror infrastructures,[29]    protecting or expanding polar sea ice and
glaciers, including the use    of insulating blankets or artificial
snow,[30][31] using pale-colors    for roofing materials and other
man-made surfaces (i.e. roadways and    exterior paints), attempting
to change the oceans' brightness,    growing high-albedo crops, or by
distributing hollow glass beads in    selected areas to increase ice
coverage and lower temperatures.[32]
Troposphere-based: for example, marine cloud brightening, which would    spray fine sea water to whiten clouds and thus increase cloud
reflectivity.
Upper atmosphere-based: creating reflective aerosols,    such as    stratospheric sulfate aerosols, specifically designed
self-levitating aerosols,[33] or other substances.
Space-based: space    sunshade – obstructing solar radiation with    space-based mirrors,    dust,[34] etc.

Carbon dioxide removal

Creating biochar (i.e. in biomass-fired thermal power plants), for    mixing into the soil to create terra preta
Bio-energy with carbon capture and storage to sequester carbon and    simultaneously provide energy Carbon air capture to remove carbon
dioxide from ambient air
Afforestation, reforestation and forest restoration to absorb carbon    dioxide
Ocean afforestation and ocean fertilization (which includes iron    fertilization of the oceans)


Why have none of these methods been implemented?
What are the risks of developing geoengineering?
We don't know, and that's why we have to develop it.
that's the thing about undeveloped sciences we don't know what many of the risks are, Its a case of knowing what we don't know. Which is one big reason to develop it, so we don't cause harm accidentally.
Lets be clear we already are geoengineering right now, by driving cars, changing the landscape, polluting the air,  we are just doing it blind. We have to research things to understand the risks, and geoengineering has barely be scratched.