So there you are, working away in Illustrator, making something that you want it to repeat a whole bunch of times for you. You drag it to the Brushes panel and you get something like this.
Perhaps your first instinct is to start searching for what elements can’t be used in a brush, and then object>expand all of those parts into things that can be used in a brush. But, you know, that starts to feel like work or something, and if you’re at all familiar with the way I use Illustrator I’m all about skipping those parts.
So instead of doing all that work, how about making Illustrator do it for us? Ever since 17.0/CC, Illustrator lets us put bitmaps in brushes. And there’s nothing saying we can’t generate those bitmaps directly in AI.
So: select all the stuff you want to turn into a brush, then do object>group, then do effect>rasterize. And now you can drag this into the Brush palette.
Looking at it up close you can see a tiny bit of pixelization going on. If that bugs you, then select your original group and visit the Appearance palette to change the settings on that rasterize effect, then alt-drag it on top of the brush in the Brush palette.
You will want to save a copy of your original art somewhere in your drawing. I usually put it on a layer named something like “construction” that I keep hidden most of the time. If you try to access the original art by dragging the brush thumbnail onto the canvas, you’ll just get an uneditable image.