All Chrome extensions are sandboxed and require the manifest to explicitly specify which site(s) it operates on and what permissions it requires to work (a-la Android apps) which means you have a guarantee the extension isn't doing anything outside the scope of the permissions you allowed it.
That's not to say malicious Chrome extensions don't exist, but when you install an extension you don't hand it keys to the entire browser and the rest of your extensions too.
Show me a Chrome extension half as useful as Vimperator
NoScript
ScriptBlock, ScriptSafe, or uMatrix
Greasemonkey
Stylish
RequestPolicy
DownThemAll
GetThemAll, Download Master, Metalink Downloader, or JDownloader Integration for Google Chrome