Unless you've written a Widevine client, downloaded from DASH, parsed MP4, decrypted MP4 samples, then reassembled the decrypted fragments, then you're really not in a position to be making this claim. I have done all the above, and the MP4 parsing was by far the most difficult part of the process, and that includes parsing Protocol Buffers for use with Widevine. The sheer volume of different box types is what makes it difficult. Over 100 types, see for yourself:
It's actually my go-to project when I'm trying to learn a new language, because the problem itself is simple enough to understand, but it forces you to learn the idioms about the language you're learning. What is the idiomatic way to represent different box types? How do you read values with specific endianness from a buffer? How do you seek through a file's contents without loading the whole 10GB movie into memory?
Matroska/WebM is so much simpler and easier to parse, you can essentially abstract it away in a JSON-like DOM (obviously without loading 1GB of data into memory) and just get what you want, it's great.
MP4 is seriously one of the easiest binary formats to parse, and even if you couldn't use an existing parser it would probably be the simplest part of whatever you were actually building.