It's pretty capable for diagrams and illustrations. At least at my skill level which is about intermediate. 🙂 I still use Procreate the most out of my creative apps but that's raster. Another vector app I've used and like is Concepts. It's a bit newer than the original iDraw/Graphic app and has some IAP for extra tools, but it's a nice one as well, and I think you can try it out for free to see if you like it.
Graphic is worth the price though, I think I paid $8.99 back when it was iDraw.