This is easily my favorite Ultra Beast. It has the most balanced, usable, and arguably normal distribution compared to its fellows,
I think that was supposed to be the point. That the Ultra Beasts (in general. Celesteela is the exception that proves the rule) are meant to be radically imbalanced. They are godly in a couple stats but the rest are barely useable at best and downright horrible at worse (On the brightside, Blissey now has a companion for all these Physical attacks :P).
In a way the stats help reinforced the Eldritch theme they have going, most normal pokemon do specialize in certain stats but they would still take or deliver a punch better than a Caterpie. Likewise they all have a rare (and unique in most cases) type combination that also leaves them with one 4x type weakness, (Notable, The subject of the thread is the only one to buck those trends, while Xirkitree is the only other UB to not have a high weakness to any type (being just Mono-type Thunder).
I have a feeling that both were on purpose, as a certain chimerical pokemon as created in response to these beings. The pokemon Type: Null, modeled on Arceus who not only has the ability to change types at will, but also has its stats being extremely balanced across the board (in relation to each other, due to being the exact same.), this makes them adatable to the Ultra Beasts.
Kartana slicing up buildings? Take a Fire Plate/Memory and a burn it down with Flamethower. Nhielego spreading deadly nuerotoxin in laboratories? Bust out the Grout Earth Memory/Plate and give that jellyfish an earthquake. Guzzlord chugging down Oceans? Simply change to Fairy and and take that sucker down with Dazzling Gleam.
To put in simple terms, Type: Null (and Silvally) was designed to be the perfect counter to the Ultra beast (though not the only Counter...unless you're Pheremosa :P, there's also the matter of how it works in practice.)
Celesteela here is an outlier because their stats are
almost equal to Silvally's (exact same BST but everything but Speed is higher), and theres no crippling type weakness for Silvally to exploit (Flying covers Electric's weakness, and its won weakness only ensure 2x damage at best as opposed to 4x like the other UBs). The ramifications are...interesting to say the least