I can't understand why people say it's marginal for deer.
I like .243 Win - light recoil, good BC, short action, etc. It works as a deer hunting cartridge, however, whenever someone wants to discuss "what's the minimum for..." there are usually caveats. The same applies here.
The cartridge has plenty of velocity, but bullet choice and performance is what makes it marginal in some hunters eyes.
If 80-87gr bullets are chosen, are they constructed for deer and not varmints? No one wants to wound game or have to chase it.
If 95-100gr bullets are used, there is a risk of overpenetration. Several years ago, I took a doe at 80 yards with a 100gr SP with a heart/lung shot. She had run off into the woods and the blood trail tapered off after about 40 yards. An hour and a half later, I finally found her. Field dressing showed that it sailed right through and didn't expand much. On the bright side, the meat damage was .243" on entry and only a little larger on exit, with a bruise about 2" in diameter!
There could be several takeaways from this experience - maybe I needed to use a tipped 95-100gr bullet to aid expansion. Maybe I should have purchased bonded or partition bullets. I couldn't have aimed in a better place. Maybe our Missouri whitetails have more will to survive! - not likely. After all that second guessing, what I chose to do is go back to a 30 caliber cartridge. It's not like I can't handle the recoil, deer hunting is usually just a single shot, and I have never had to track more than 30 yards or so. If my experience is any indication, that is why some hunters consider .243 Win marginal/minimum for deer.