everything you have against thunderbird?

I noticed that thunderbird have some unfortunate
default search engines. And the remove search engine button can
not remove them. Also I have come to wonder if
thunderbird comes with some questionable telemetry
properties. on privacy, security and surveillance
what do you have against thunderbird?

Is icedove nothing more than a free software version
of thunderbird? Or has icedove countered what
seems to be thunderbird surveillance efforts? If
so is there documentation on how icedove
has been modified? Thank you.