What are the main risks?

Your money is at risk, and there are never any guarantees that you will get the returns or rewards indicated. Debt investments tend to carry lower risks than equity investments for example, but really all investments have different risk profiles and you should always read offer documents in full and do your research before investing. The opportunity you put money into may operate longer than the crowdfunding platform you use, so you should also check whether they abide by the UK Crowdfunding Association’s code of practice.