I don't see it as a problem, necessarily, that the legends work asymmetrically in terms of what counters them. To some degree this is a feature of them using different slots for their field effects, vs having Miraidon summon rain or something. It's nice to have legends that play differently vs being physical/special clones of each other (gen 5 and 7) or one just being obviously superior ( gen 8). I liked the gen 6 legends in the way they had identical stat spreads and abilities but mechanically felt the most dissimilar of any legendary pair since gen 2.
Advantages of Koraidon:
- Effective third STAB which is given a larger boost
- Not stopped by moves that remove terrain (notably including Defog -- I can imagine hazards being used less in SV ubers on teams with Miraidon because your opponent's defog is inherently stronger when it removes terrain aswell)
- Stronger boost provided to other mons, generally Sun provides more useful support
- Effective double resist vs water and removal of rain, allowing safer switchin against Kyogre.
Advantages of Miraidon:
- Field effect provides direct boost to STAB
- Not trolled by one of the most common restricted Pokemon, nor by any other Ubers-viable Pokemon unless Lele starts seeing use.
- Less vulnerability to status (immune to sleep and largely unbothered by burn)
- Not forced to get into a weather war if your opponent tries to use a rain, sand or hail team; terrain teams are largely not a thing because there just isn't the support there.