There's APIs for anything you'd want to do, and they tend to work well. The pricing is competitive within the industry. The documentation is decent and the reporting, though somewhat limited, is still decent enough to suffice for most applications.
The fundamentals are there, and Dyn should work well for most applications that require purely transactional emails.
Honestly, most of these issues could be ignored or excused if their support wasn't so bad. They have no support ticket ownership, which makes getting a complicated issue resolved very time-consuming and frustrating. Every response is from a different person, requiring you to re-explain yourself many, many times.
This feeds into another problem: there's a lack of transparency regarding their deliverability from their shared IP pools. They end up on lots of blacklists, and don't seem to have the baseline reputation that other providers do, but this isn't obvious until you start sending through them. When you try to reach out to their support team for an explanation or more information, well, see above.
A more minor gripe, but some parts of their web UI are mind-blowingly slow, most notably the sub-user list when there are many of them. I've actually written mini apps for internal use just to leverage their API speed over having to deal with their web app's slowness.