I've worked for such a place. Off-the-shelf solutions are trying to be everything to everyone and you can end up customizing the crap out of them to the point

I've worked for such a place. Off-the-shelf solutions are trying to be everything to everyone and you can end up customizing the crap out of them to the point where it can become more onerous than just building your own. The place I worked was also print-on-demand service and had hundreds of thousands of SKUs as well as allowed customers to make custom products and we also hosted some peoples' shops. Shoehorning that into a custom solution was painful. I work at a very similar place now that uses Shopify. Managing that many SKUs on Shopify is crazy painful. The thing is is that custom ecom solutions really aren't that hard. The off-the-shelf ones are complex because, as stated above, they are trying to be everything to everybody.