Are non-OEM batteries not optimized to work on a specific model and thus resulting in poor battery life?

I am looking to replace the battery in my Samsung Galaxy S8. Amazon shows many available batteries, ranging in capacity from 3000mAh to 4350mAh, all about $20 or less. The one and only authorized Samsung technician in my neighborhood tells me that none of those batteries is optimized to work with the phone, that any of them would result in very poor battery life, and that I need an OEM battery, a capacity of 3000mAh, for $99 (plus labor of course). Are non-OEM batteries not optimized to work on a specific model and thus resulting in poor battery life?