You said it, a timer, which is something Ursaring has as well, and arguably even worse than Swellow because Steel types and Rock types will only take major damage from its Close Combat. Rock and Steel types are still typically faster than Ursaring, such as Lucario, Kabutops, Tyranitar, and some are not even weak to Close Combat such as Jirachi, Metagross, Skarmary, and Aegislash.
Further, most naturally have a high attack stat, meaning Ursaring will likely not get to hit them with Close Combat before taking the fall from their STAB attacks. Even if Ursaring is Jolly, while Tyranitar is Adamant, and thus giving Ursaring the Speed advantage, Tyranitar/M-Tyranitar gains Sucker Punch and can hit it before Close Combat, also add in the damage from Sandstream, and basically can KO it, especially because Ursaring will be affected by Toxic or Burn orb damage already.
So Close Combat is less of a factor. Lopunny also uses up a Mega Slot, and when it comes to Ghosts, Swellow's Dragon Ascent allows it to hit them for 180 base power off of higher Attack, which more than makes up for Lopunny's slightly higher Hi Jump Kick at 195 base power.
Gengar also takes more from a Guts Swellow Dragon Ascent than from a Lopunny's Return, and as a Poison type, we all know it resists Lopunny's stronger Fighting STAB. Plus, can't foes use Protect on Hi Jump Kick to force recoil?
The biggest reason for Swellow over Ursaring, is letting it KO with Facade. Since it is naturally fast it won't have to rely on Extreme Speed to hit hard and first. The lower attack compared to Ursaring is made up by using Facade, which is almost double the base power of Extreme Speed.
Keep in mind with Swellow's typing it can use Swords Dance, or Copy Cat, or even Me First as rock and some steel types are notorious for using Ground or Fighting attacks for coverage, so Swellow switching in on Earthquake Metagross will be in for an Earthquake themselves.