One of the driving forces behind the neonicotinoids theory is a toxicologist called Henk Tennekes, who used to work as a consultant for Solvay, Roche, Monsanto en Sandoz (now known as Syngenta). Basically he claims that the toxicity was underestimated as it grows with time, the damage to the nerve system in insects is irreversible.
The pesticides also go into the surface water, where they found a link between higher surface-water concentrations of imidacloprid - a specific neonicotinoid - and a decline of birds that feed on insects.
Perhaps a better source in English, the EU policy advisory report from the European Academies Science Advisory Council from 2015 on Ecosystem services, agriculture and neonicotinoids.
https://easac.eu/fileadmin/Reports/Easac_15_ES_web_complete_...
Most neonicotinoids are now banned for outdoor use in the EU: https://ec.europa.eu/food/plant/pesticides/approval_active_s...