When Shiny Genesect was distributed, it came with event moves it can't learn any other way - Shift Gear and ExtremeSpeed. A regular, non-Shiny Genesect reveals that it cannot have either of those moves, and since that event was in Gen 5, it's not missing out on any tutor moves or anything else it couldn't otherwise access. In the past, it was Nature-locked to Hasty, but with the addition of Nature Mints in Gen 8, that restriction no longer applies. On top of that, Shift Gear Genesect with Leech Life and Iron Head is a very respectable option - not being Shiny instantly reveals you're not that set. Of course, you can still be Shiny and not be forced into using those moves, so a Shiny Genesect could be your typical Choice Scarf set with no event moves at all.
So it's always optimal to use Shiny Genesect in Generation 8.