All extremely good points (personally #4 tends to run my life :) )!
I have done a couple Shopify stores before, and while your points are all valid, it comes down to what you are trying to do…
If the goal is to create a startup/shop quickly and test the market viability, those reason’s start to become less important as the goal is to get something running sooner rather than later.
Starting a shop, using one of the free themes and setting up recurly (for example) is a couple hour task. I don’t want to assume anything about your skills, but coding it myself would take more than that :)