Fini also has 25 more base Speed. Even with both usually being uninvested, Fini outspeeds some very common Pokemon like Incineroar, Aegislash and uninvested Ttar. I rather have that Speed + better bulk + Terrain + more unpredictability than an accurate Muddy Water + higher power + Energy Ball. Energy Ball is useful for rain teams and Gastrodon, and Gastrodon usage is on the rise; could fit in on a Specs set. Though it may not be worth the slot over a STAB move and Ice Beam given that Specs Moonblast works fine against most opposing Water-types. I rather use Hydro Pump over Shadow Ball as it hits harder with STAB and has more general use like OHKOing Koko with Specs. Perish Song and Encore are nice support, though generally not as useful as choosing between Taunt, Heal Pulse, Haze, Misty Terrain Swagger, and Soak which can accomplish some of the same goals like punishing set up. Misty Terrain is also increasing in value with Kommo-o's increasing usage.
Primarina could be more useful for Perish Song teams, but it still has to compete with Perish Song Toed and Azumarill, who have better mixed bulk and also have Encore.
Primarina has access to encore as well, you know. As well as Psychic, so its not totally walled by Amoonguss and Venusaur. I haven't tested it myself but here are calcs: 252 SpA Life Orb Tapu Koko Thunderbolt vs. 252 HP / 0 SpD Primarina: 143-172 (76.4 - 91.9%) -- guaranteed 2HKO ( This is without Electric Terrain. )
252+ SpA Choice Specs Primarina Moonblast vs. 4 HP / 0 SpD Tapu Koko: 165-195 (113 - 133.5%) -- guaranteed OHKO
252+ SpA Choice Specs Tapu Fini Moonblast vs. 4 HP / 0 SpD Tapu Koko: 136-162 (93.1 - 110.9%) -- 62.5% chance to OHKO
252+ SpA Life Orb Primarina Moonblast vs. 4 HP / 0 SpD Tapu Koko: 142-169 (97.2 - 115.7%) -- 87.5% chance to OHKO
252+ SpA Life Orb Tapu Fini Moonblast vs. 4 HP / 0 SpD Tapu Koko: 118-140 (80.8 - 95.8%) -- guaranteed 2HKO
252+ SpA Choice Specs Primarina Hydro Pump vs. 252 HP / 4 SpD Mawile-Mega: 156-184 (99.3 - 117.1%) -- 93.8% chance to OHKO
252+ SpA Life Orb Primarina Hydro Pump vs. 252 HP / 4 SpD Mawile-Mega: 136-161 (86.6 - 102.5%) -- 18.8% chance to OHKO
252+ SpA Choice Specs Tapu Fini Hydro Pump vs. 252 HP / 4 SpD Mawile-Mega: 129-153 (82.1 - 97.4%) -- guaranteed 2HKO
4 Atk Aerilate Salamence-Mega Double-Edge vs. 252 HP / 0 Def Primarina: 144-169 (77 - 90.3%) -- guaranteed 2HKO
252 Atk Aerilate Salamence-Mega Double-Edge vs. 252 HP / 60 Def Primarina: 157-186 (83.9 - 99.4%) -- guaranteed 2HKO
252+ SpA Choice Specs Primarina Hydro Pump vs. 0 HP / 0 SpD Charizard-Mega-Y in Sun: 134-158 (87.5 - 103.2%) -- 12.5% chance to OHKO
252+ SpA Charizard-Mega-Y Solar Beam vs. 252 HP / 0 SpD Primarina: 156-184 (83.4 - 98.3%) -- guaranteed 2HKO
252+ SpA Primarina Oceanic Operetta vs. 0 HP / 0 SpD Charizard-Mega-Y in Sun: 156-186 (101.9 - 121.5%) -- guaranteed OHKO
252+ SpA Primarina Oceanic Operetta vs. 28 HP / 4 SpD Tapu Lele: 159-187 (106.7 - 125.5%) -- guaranteed OHKO
252+ SpA Primarina Oceanic Operetta vs. 0 HP / 0 SpD Metagross-Mega: 165-195 (106.4 - 125.8%) -- guaranteed OHKO
252+ SpA Primarina Oceanic Operetta vs. 252 HP / 4 SpD Mawile-Mega: 186-219 (118.4 - 139.4%) -- guaranteed OHKO