One of our clients got a huge international eCommerce website. We advised them to use the hreflang attribute since most of their product pages exist on all of their web shops. However, there are also quite a few products that are not listed in every single web shops. Now if they used the hreflang attribute, they might have a lot of useless code pointing to non-existing pages in the other shops.
They said it might be quite a hassle to fix this. So I am wondering: What might the impact be on Google and other search engines, if they just flat out used the hreflang regardless of whether or not the respective product pages exist on the other shop?