they did answered!
this is the old same concern with linux, windows and virus
in windows a unknown app not being detected by the antivirus will basically do a party in your pc, virus, rootkit, malware, spambot, fake ativirus scam and other tricks that a windows machine let happen for his insecure nature, everyone, even under user account can install and run almost every app they want, not the case on linux
as you know, linux will require root privileges for important stuff, this kind of apps are powerful but encapsulated on linux
a direct attack to your machine form this kind of apps is not common but not impossible
it is the same situation with firefox addons, you don't know who developed it, don't know if it does more than what you know or need
this days is hard to trust on anyone isn't?
for me, i loaded two google widgets and plasma crashed so my advice for you is don't rely on google widgets but if you have a real use for a specific widget, test it, monitor it to see what it does
other than that, i don't see how to trust on a app i can't see the source code or find someone who guarantees the security state of the app and to be honest, i don't like google in general