The primary risk of dealing with any small business is that if key people are not available, then performance may suffer unless they've got risk mitigation strategies in place. In my own business we get around this problem by having good relationships in place with colleagues who are able to assist if I'm sick / on holidays / snowed under with existing work, etc.
Terry has mentioned death and legal problems as a threat. We already have contingencies for these events. Legal problems would likely have a greater impact on my clients than my death. Neither event would cause more than a 24 hour delay in getting a loan approved.
I've also dealt with clients who ran into problems dealing direct with a bank because the person taking care of their loan got sick or went on holidays. In many ways small businesses deal with these events better than large ones, because they are more likely to identify it as a significant problem.
Most brokers have access to a similar number of lenders. Almost all brokers operate within an aggregator which is by no means a small business and they get all sorts of support (including access to lenders) through their aggregator. In reality this is no different than going through what you might consider a larger broker.
Consider Mortgage Choice, what you might think of as a larger broker. In reality they're all franchises where each franchise is a small business that started with a single broker working out of a spare room. The main difference of these guys from any other broker in their business is they operate under a known brand with a multi million dollar advertising budget. Beyond that there's no real difference.
Smaller brokers also have more flexibility. I've got access to more lenders than anyone operating under Mortgage Choice could ever enjoy.
I'm reasonably certain that the business of every broker that posts regularly on this forum meets the legal definition of 'small business'. Some operate out of a small office, others out of their own home. Some have staff, some outsource certain work and some do it all themselves.
I also know that some of the brokers here would be considered at least in the top 5% of brokers in Australia. It's likely none of the regular contributors would be below the top 20%. If you've found a broker here that you are comfortable with and have confidence in their advice, you're probably in very good hands.