Magento newbie, but developer last 30 years (mostly backend). Taking my e-com from Prestashop to Magento but can't decide about the frontend. Honestly don't have that many requirements on special-special, just a plain storefront. Actually Luma with a couple of modules would be ok. Thinking more of what would be best long term...
If there is a possibility to develop your own theme by extending Luma, it would be great.
From my experience, the majority of themes come with:
1. you'll feel like you're in hell while trying to modify CSS because majority of themes have code-generated CSS (for functionalities like changing font/color of various items from Magento Admin).
2. tightly coupled code - if you change something, you'll be probably in trouble
3. too many features which you do not need
4. performance bottlenecks because of poor development practice (at least what I saw)
5. potential security issues which could come with extensions which are not validated by Magento
Again, this is from my experience.