Is it space efficient? (Compared to whatever you're prepared to license and run on the cpu) Not shipping a codec to save app size but sending larger media doesn't help the user much.

Are all implementations secure? If you have to predecode to verify the file won't trigger buffer overruns etc, you've written half of a safe decoder, and maybe you should just include the rest of the owl. Media decoding is a bountiful area of security vulnerabilities... which brings risks to both using the system apis and using your own, but at least you have some control over your own.