Monogame is not so much a game engine as it is an I/O framework. It's got primitives to get input, to draw on screen and output sound and really barely more than that.It's much more fair to compare it to SDL than to a full-fledged engine like Unity. It has some of the elements you need to build an engine, like asset management and stuff, but it's really bare-bones and expects you to design all the higher level logic yourself.

And honestly, that's fine. It's 2D only (afaik) and designing your own main loop is a piece of cake anyway.

The only real problem I have with Monogame is that it's cross-platform only really in theory. In order to build your games for multiple platforms you need to use different forks of the codebase, or at least that's how it was when I tried it.