SimpleSync » Sync by SKU

Two stars for Harold actually responding to emails. Other than that, SimpleSync did not work. We have a very basic setup - 2 SKUs, 3 variants each, no custom code apart from a warehouse that sends Shopify the physical count of each SKU once an hour. SimpleSync failed to work for us - it decremented only the variant sold, and a few minutes later, the inventory went right back *up*. Harold is adamant that his app works, and the fault is in the way our site is setup. I installed Connected Inventory by InsCoder and it worked right out the box. So there's an obvious difference between how the apps are built - one works, the other doesn't.

This app was not very useful to me. I sell handmade notebooks. I have several different cover designs, each of which can be made with 6 different page options inside. I have virtually unlimited supply of pages, but only limited number of each cover. So if I have 5 covers left, I need to be able to link my page variants so that all 6 options are still available to customer, but no matter which combination of page options they choose, they can only purchase a total of 5 books in that design.
This app does sync inventory across variants, but if I set my stock level at 1, it will assign a value of 1 to each variant. If a customer orders just one of the variants, the app changes all the variant stock levels to 0 after the order is processed. So it appears to work. HOWEVER, it only syncs the inventory after an order is processed. It does not prevent a customer ordering two different variants in the same order. Because according to my store, I have 1 of each variant available. Even though I only have one cover left, a customer is still able to purchase two different books with that cover, meaning I can't fill part of their order.
Harold tried to explain to me that syncing the inventory in the cart, rather than after order checkout would slow things down and result in fewer sales. But the way this app currently works, it means I have to make some product variants unavailable when my inventory quantity gets to low numbers, meaning fewer choices for my customers and that results in fewer sales for me.
This app might be sufficient for some businesses, but I need a way to completely manage inventory at the product level, not the variant level, and this app just doesn't do that 100%. I'm yet to find an app that actually does.