Opencart was a great platform, we moved from there. I have been seriously considering moving back. The IG/Shoppable posts is a bit more of a headache to integrate than it is here, but there are so many features built in that you have to pay for here.
It will be smooth if you plan it properly. There was no down time. But it will also depend on what alternative solution you are going for. I simply build the stores for my clients with the requirements they asked and simply didn't point the domain to the new stores until they worked as they wanted it to work and until they were trained on how to use it.
One of the clients had a shipping integration set up for shopify that automatically gave customers a list of shipping options through a shipping broker, we just altered that to talk to their new platform and away we went. The shopify solution required a convoluted way to get volume dimensions into the products as metafields, before we could use this custom integration but now we don't have to worry about this.
This subject has been marked as “solved” by Shopify in the community page. This has not been solved and I feel they have no incentive to fix it. Shopify wants us to pay for their third party apps only. Please start turning in request for this change. I have sent many to the Shopify Team and I plan to keep sending them until someone gives us another option.