You'll find most brokers have your funds held in a segregated trust account which removes some of the risk should the company 'go under'. All normal banking rules apply to brokers here in Australia which endeavors to protect investors money held on deposit.
Some brokers will suggest that all free equity is held in a trust account whereas money held in open positions may be challenging to get back should something bad happen to the company.
Most of them pay a very average yearly return unless you are willing to put up say $100,000 or more on deposit.