There actually are Hubitat integrations for both Nest (official from Hubitat, but you need your own Nest Dev Account currently) and Logitech Harmony Hub (via my community developed integration for Activities).
This doesn't mean that I would recommend removing the native integrations from your Google Assistant/Home devices though. Those are very nicely engineered and supported official integrations. It can get confusing when the same devices are being presented to Google Home or Amazon Alexa devices from multiple sources. You just need to remember how it is all "wired together" between the various systems to avoid confusion and possible endless loops! 
