Re: How bad can it be?
As an example, read item 47.
The principal issue is that the moment you have created an account at Google, you have effectively given them permission to not only gather data on you, but use such data in a manner that is so badly documented that Google is in trouble with the EU Justice Art 29 Working Group on privacy, and has received a letter signed by 27 (yes, twenty seven) different countries asking them to ensure their privacy policy becomes compliant with EU law (which is isn't, not by a long shot).
More specifically, the mapping app records what places you look up, and it can track where you are - all of which gets recorded for whatever use Google finds for it. Basically, you carry a stalker in your pocket if you log in to a Google account..
You will hear a lot of blabla from people claiming that Apple does the same, but there is one, rather crucial difference: Apple doesn't generate revenue from user data, it's a hardware company with services, whereas Google exclusively makes its income from data derived from a somewhat overly creative approach to user privacy..