When I was trying to get a shiny Zygarde using the Reassembly Unit in a world with the shiny rate config set to 2.0 (50%), I didn't get a shiny from dozens of tries, which would be highly unlikely if the rates were really 50%. I tried 10% forme, 50% forme, and 50% forme with Power Construct and nothing worked. The other Pokémon spawning in the overworld however, were 50% shiny as the config was set to. I'm additionally concerned that Zygarde may in-fact be shiny locked from the Reassembly Unit, although I haven't done enough shiny rolling to be sure of this.