I had the simple task[2] of encrypting bytes with AES using the library. But (as detailed in the post), they didn't expose a simple, "give us plaintext bytes, give us key bytes, give us options, we'll spit back the ciphertext bytes". Instead, you have to delve into the irrelevant details of constructing mutable buffers (which require special care in Rust), so it can read from and write to them, and without even any good examples in the docs for how to construct those buffers.
It seems designed from the perspective of "I have extremely limited memory" -- which is fine, but why not have a simple wrapper on top of that for the common, non-embedded use case, and some example code?
[1] https://news.ycombinator.com/item?id=18943940
[2] for the Matasano/cryptopals challenge. Interestingly, I found that the hardest parts weren't figuring out the vulnerabilities, but in implementing a spec (every writeup was poor) or getting "off-the-shelf" software to behave as expected so I can have a utility function for it and forget about it from then on!