I think the .300 Wby developed a bad reputation amongs some PH's / Outfitters when hunters showed up just recently having bought them. Because they were just recently bought or perhaps rarely shot, the hunter was not used to the recoil, and were flinching at it. Now you've got a bullet traveling at that super speed but not being placed in the vitals. Lots of long follow ups versus hunting for the next critter is not what a PH wants to be doing...
I think Phil is right on here. Too many newby hunters with flashy new Weatherby rifles that they either were afraid to shoot and/or hadn't shot much, and especially years ago, with bullets that wouldn't hold up to Weatherby velocities.
After 40 years of admiring Weatherby rifles, in 2009 I finally bought a .300 Weatherby Vanguard. I took it on a PG hunt in South Africa in 2012 and it made one shot kills on all of my animals from a Klipspringer at 314 yards to a beautiful Sable bull. My handloaded 168 grain Barnes TTSX bullets also performed perfectly.
Last March I took my .300 Weatherby to New Zealand where again the 168 grain TTSX bullets made one shot kills on 4 of the animals that I shot there, including a great Red Deer stag.
I've also used my .300 Wby to make one shot kills on two Montana bull elk and a reportably hard to kill west Texas Aoudad, also with 168 gr TSX and TTSX bullets.
When I bought my .300 Wby, I immediately put it in a custom stock that
fits me, had a KDF muzzle brake installed on it, and put a mechanical recoil reducer in its stock. Its recoil doesn't feel any harder that the recoil from my .308 Win. For the past year or so, I have shot my .300 Wby almost every week at our local range--5 shots standing or sitting at a 100 yard gong, and 4 shots prone at the 430 yard gong.
For over 20 years, my favorite elk bullet was the 180 grain Nosler Partition at .300 Win velocities. The bullets that I recovered had all expanded back to the partitions, shed the lead from the front portion of the bullet, and weighed about 100 grains, or had retained about 56% of their original weight. I have only recovered 6 TSX or TTSX bullets from the animals that I shot with them. All of these bullets had expanded to look like the mushroomed bullets in the Barnes advertisements. Two of the TTSX bullets had only lost their plastic tips for a retained weight of 99% and two of the TSX bullets had a retained weight of 100%. The other two bullets had hit shoulder/upper leg bones and had lost one and two petals, but still retained over 85% of their original weight. This is why you can go down one or two bullet weights with Barnes bullets -- you still get great penetration and will have a high retained bullet weight.
Most of the animals that I have shot with the TSX/TTSX bullets from my .300 Weatherby resulted in complete pass throughs, including one of the bull elk, the Sable, and the Red Stag. If you use these bullets, make sure there is not another animal behind the one you are shooting at.
My only regret with my .300 Weatherby is that I didn't buy it 40 years ago. It has quickly become my favorite rifle.