The thing about Terrakion that makes it managable is that it only really gives a crap about 2 moves (Stone Edge and Close Combat) so prediction is key. Most Terrakion are choiced anyway, and thinking about relative power, it really isn't that much more hard-hitting than other things, despite the excellent dual STAB.
Also, just because something cannot switch in does not mean that the Pokemon is impossible to counter, as row power doesn't mean ALL that much. For example, in NU we got out buddy Charizard with Sun Boosted Solar Powered Fire Blasts that, with Specs, 2HKOs all but 2 Pokemon in OU (Eviolite Muchlax and Snorlax) that aren't immune without changing the weather, including Chansey, Milotic, Blissey, and even Kingdra (4x resistant!). However, he is NU (though there are other reasons, it still is a point). Latios's Draco Meteor is almost impossible to survive without a Steel type/Special Wall, he isn't weak to Stealth Rock AND is immune to Spikes. Haxorus, Salamence, Dragonite, Tyranitar, Rhyperior, Darmanitan, and others stand out as Pokemon in OU or UU that hit very hard as well. Even then, a lot of those Pokemon have counters that cannot switch in, but instead have to revenge kill them (try to switch a Mamoswine into a Haxorus's Choice Banded Outrage; it doesn't work).
Scizor's Bullet Punch can OHKO it and it is hit hard by rain-boosted moves, too, which are two SUPER common things in the tier. Gengar with disable also walls most Terrakion cold. Latios with Psyshock always KOs Terrakion. If it is Choiced, a resistor like Jellicent, Breloom, Starmie, or Gliscor can easily come in and since Terrakion CANNOT switch moves, it has to leave (Scarfed Versions are weaker than that). Toxicroak restists its dual STABs and can outstall it in the rain. In lower tiers, Tangrowth, Slowbro, Golurk, Nidoqueen, and Claydol are great at it too. There are other ways of dealing with it too. Really, Terrakion is a scary offensive force, but it can really only do one thing well and it is not versatile at ALL and can potentially be revenge killed, since its Speed has its limits too.