The way you are asking your question doesn’t follow WPSE guidelines. You will get a lot of opinionated answers which is not what we look for here.
Try to come up with more context with your question.
- What are you building your theme for?
- Who’s your users (single client or public/masses)?
- What problems are you trying to solve?
- What tools are you considering to use to solve the issue?
- Are you planning to build extensions/modules for your theme
- Do you wish to separate some core functionalities from design or offer your client to modify design without affecting core functionalities (parent/child designs)
- etc.
The worst method to build any program is to not be structured in your program logic and have functions all over the places (which would be a problem not specific to WP theme dev), repeating yourself, etc.
If you came up with a question like “I need to do so and so, is it better to use CPT or custom field to solve my issue” then it would be easier and more productive to answer that kind of question. We could then provide the pros and cons of either solutions.
There is all kind of situations, problems, clients, contexts that can affect the way you build your software. Sometimes, because it works it IS the better solution. But generally speaking you try to be as structured and concise as possible keeping your stuff readable/understandable for other devs and you (would you still understand your code if you came back to it, 6-12 months from now?)