However add a little complexity here or there and it helps to have someone who knows what they're doing to guide you through it. Its a self assessment scheme, like tax, anyone can lodge their own return and the ATO will pay out with only cursory checks.
The real danger lies when you get audited, it can be especially dangerous for startups who have been claiming a few years, ignorant that they have not been claiming properly and are faced with an Audit by the ATO for multiple years of tax refunds. Generally once you add penalties to the tax refunds they must now pay back, the amount will end most startups.
There are many reputable consultants in this space who are capable and cheaper than the big 4. I would suggest sticking to the big 4, Tier 2 Accounting firms and Boutique R&D Tax Firms. They should be registered tax agents and generally you would want to know if they have successfully navigated Ausindustry Reviews and ATO audits before. Avoid suburban accountants like the plague for these things.
The credit from the ATO comes in the form of a check, but it's a prepayment against future tax obligations (you lost the tax-loss carry-forward you'd otherwise be able to claim to offset future profits). It's very useful for a startup since it's non-dilutive funding which can be used to get a lot of work done.