>>43936731On top of what was already mentioned, Articuno, Silvally, Guzzlord, and Electivire are all Pokemon that are all embarrassingly bad compared to their relatively high stat total.
Entei was also extremely terrible until it got Sacred Fire and Extremespeed.
>>43937536That pretty much sums it up. Poor stat spread and only Moonlight/Morning Sun for recovery is ultimately what keeps it down. Originally people assumed it'd be a lot better if it weren't for its abysmal movepool, but it finally got good moves in USUM, so it turns out that Necrozma is just bad.