Say I have designed a simple (say less than 10 parts) project for a Pi and I want SE users on here to check my schematic for flaws. Is this on-topic?
Specific questions about the design, or about the best practice with regard to some particular aspect are fine. But just saying, "Here's what I'm doing, is it okay?" or some variation thereof is too broad.
If you are interested in review, it implies you are uncertain about some things. In that case, you should ask about them directly. Uncertainty is normal and inevitable, but you must be able to deal with it yourself well enough to isolate, focus, and prioritize so you can make the best use of resources such as online help by asking clear, concise questions. Everyone is excited about what they are doing and no doubt would love to have a team of experts pour over it and contribute. However, this is not very feasible, and we are not a discussion forum. There is a point to that, and if you make an appropriate effort you should often be able to get the information you need.