I'm looking to start a service where I collect Windows events from consumer computers over an SSL link (https) but I want to make sure that extremely sensitive information wouldn't be contained in the data (especially if a hacker got a hold of the information)
We can't answer that. Applications can put whatever they want into the logs; you could write a key logger that logged every keystroke on the system into the event log, or a web app that dumped every user's plaintext password into a log entry.
You also haven't defined what "extremely sensitive" means to you. By default, Windows isn't logging anything that most people would consider "extremely sensitive", but in certain environments, account names of users or IP addresses of systems are considered extremely sensitive.
Logs are just as sensitive as the data that's in them; that's the best answer we can give you.