Nothing to say negative about a 243 when using quality bullets. They may have a flatter trajectory - all depends on bullets, velocity in your barrel.
As to exits, some of you may have not tried say the 95 Ballistic Tip, Barnes/Nosler mono's, Partitions, or Swift bonded. Other's will also, but the 243 is far more gun than often credited. I could see a niche on varmints thru coyotes, Pronghorn, and yes, even Deer/hogs.
That said, I have run an equal # of 6.5s. But my longest deer kill was the 243 Short. Aka BR
400 yds ranged, using a 105 amax........exited offside after destroying both lungs.
The 6mm Creedmoor will find a niche......but it will take time.
Oh, that 95 Ballistic Tip.....it exited every deer in both BRs and 243s....last one busted both shoulders broadside on an 8 pt WT buck at 100 yds. I call that great performance, at a distance bullet retention will only be better. During R&D it was used to take much larger game, IIRC, Elk.
No flies on the Creedmoor, both very capable rounds. Bullet choice not quite as fickle, most are game bullets and do well at modest CM speeds.