Over the years I've written extensively about the downsides of convertible notes for startups (here, here and here). (inc.com)
One advantage of issuing convertible notes is that it does not force the issuer and investors to assign a specific value to the company at a stage where insufficient information is available. (seedinvest.com)
The form of investment is dependent on the company's relative maturity with seed stage investments typically structured as convertible notes while early stage companies issue preferred equity in exchange for investor funds. (microventures.com)