The biggest problem with motherboard repairs is that there are a lot of unrelated components on a motherboard. For example, the wifi chip set is not related to the power management components. That means we could replace the wifi chips on a motherboard and a year later the power management may fail for completely different reasons. So while both are located on the motherboard, we wouldn't warranty both.
Another issue with motherboard repairs is it can be hard to tell when new damage is inflicted on the device. For example, if we fix the power management system and then someone plugs the device into an unauthorized charger and blows the circuits again, this wouldn't be covered by our warranty (we don't cover new damage). However, it can be really difficult to identify this kind of new damage.
For these reasons, we have decided to offer a 6 month warranty that covers ONLY the part of the motherboard we repaired. Going back to our wifi chip repair example. If we fix the wifi chip and then 3 months later the power management system fails, that would not be covered under our warranty since it's a different part of the motherboard.