2 main problems for shops which have PARENT products:
1.) Variations: You cannot choose variations for which you want to have the upselling product. For example: a green shirt was added to the basket of a PARENT which is t-shirt. Logically you could upsell a green pullover or green socks but that is beyond the scope of this plugin.
2.) Problems using parents as upsellin products: Then just used SHIRTS as trigger to upsell SOCKS & PULLOVERS. Its not ideally the embedment was successful but then there were several STOCK PROBLEMS when i tried to test myself. So if I want to choose the green socks - from example above - it showed "products are not available" EVEN THOUGH the products are on stock.
So there are many bugs and for me the plugin therefore is just useless...