Let's see. Most custom barrel makers all say its incredibly hard on throats. I agree, but of course who cares about my word.
243 cases stretch more than any other - ridiculous for such a small case
They are never offered with a proper twist, offering marginal stability.
I want to say it sucks to reload, but that isn't true. This is in the middle. There are far worse, and far better. Reloading isn't just cranking a handle, it includes prep work AND tuning. A 300 RUM is far worse, for tuning as an example.
Smaller cartridges (6XC, 6Creed, 6GT) easily exceed its ballistics, as they are far more efficient AND readily take higher BC bullets, and can readily handle any bullet the 243 can do and does dream about. It *could* have been great if it was built like the 6 Super LR, but it wasn't (still using magical 308 brass).
Even near the time of printing, the (now) 6mm Rem and the 6.5x55 easily did everything better.
No, the 243 is the most nostalgic and popular round. The defense is always sentiment and popularity. Almost anything else makes a better rifle. I own a lever 25-20, but my sentiment doesn't make it a great cartridge.