But Starmie is not game changing, and it can't really beat Scizor, Tyranitar and Ferrothorn as you say. Ferrothorn easily shrug off Hidden Power Fire in rain, Scizor only fears Hidden Power Fire and rain-boosted Hydro Pumps, and bulkier versions of Tyranitar can sponge unboosted Hydro Pumps in sandstorm. Both Scizor and Tyranitar can also pursuit Starmie.
Reflect Type Starmie beats all of the Pokes you just mentioned, except for Tyranitar with Superpower. RT on Ferro, Scald it until burned. RT on Ttar, Scald until burned. You can also Spin on both of them. RT Scizor and their Pursuit/U-Turn doesn't do much.
Also, without a Psychic-type attack, Starmie needs a rain-boosted Hydro Pump to OHKO Gengar, and Hydro Pump has shaky accuracy. With the right EV spread, Jellicent is not 2HKOed by Thunderbolt. Its tipying is not also that amazing; it has crippling weakness to U-Turn and Pursuit. Not to mention that most Starmie sets have some flaws that prevent them from being staples for most teams.
Almost all Pokemon have "crippling weaknesses". Ferrothorn is weak to STAB Close Combat and the ever common Heatran completely destroys it most of the time. Dragonite is weak to Dragon moves flying around everywhere and is 4x weak to a priority move, not to mention being SR weak. With RT, Pursuit isn't a problem in most cases. LO Thunderbolt on Jelly switching in 2HKOs most of them. Now, I know you can't run all these moves on Starmie, but the fact is it is so diverse that you never really know what counters it on your team.