Though Partner Eevee isn't in Sword and Shield, it would be, overall, better than regular Eevee in 2 ways if it DID exist in Sword and Shield.
Assuming Partner Eevee WAS in Sword and Shield, it has:
Better Stats than Regular Eevee:
Regular Eevee stats:
Sp. Atk: 45
Sp. Def: 65
Partner Eevee stats:
Sp. Atk: 65
Sp. Def: 85
Better Moves than Regular Eevee:
Partner Eevee can learn moves like bouncy bubble and sizzly slide. They were extremely good moves in Let's Go, placing Partner Eevee in OU.
(These Would Likely be Staying in Let's Go even if Partner Eevee was in Sword/Shield)
Partner Eevee allowed the player to fly in Let's Go and also was the only user of HMs' replacements, while regular Eevee couldn't use them. Also, Partner Eevee gave the player valuable gifts.