I have been using one of their top end routers for nearly a year. Have stock fw installed and chosen not to flash with openwrt. I’m privacy conscious but figured with the amount of customers and forum space dedicated to their products someone would have noticed any funny business by now. Router works great and have had no problems over several updates.

Any battery issues?

most University Libraries have “guides”. Simply google " guide". eg Stanford Library engineering guide. Result like: [(https://guides.library.stanford.edu/aa)] From there use any free local library access you might have to get the details.