A re-read for some maybe: Boddington on the 270 WSM.
https://www.craigboddington.com/endorsed-outfitters/hunting-tips/my-favorite-new-magnum
The WSM development had realistic goals in mind:
1. Create a modern beltless magnum, headspaced from the shoulder, not the belt.
2. Make it short to have an efficient powder column like the extremely uniform PPC.
3. Enable magnum performance in a short action rifle.
4. Create Buzz and excitement. (Like your marriage counselor says that you should.)
I post that for the benefit of those who were not around when these were talked about.
I never had a 300. Bought the 270 WSM on a whim and was very impressed.
What are the down sides?
This depends upon your priorities and preferences.
The WSM is going to give you around 250 fps. more speed and with it, energy, vs. an '06 based round. However this will come at a cost of around 7.5 grains of powder per cartridge. Cases are also expensive. Your rifle may hold 1 or 2 less of the horizontally challenged little cases. My Kimber holds 3+1. Some others hold 2+1. Cycling: multiple rifles do not like to smoothly cycle the fat little cartridges. I have never had a problem in the field though. When your adrenaline is pumping, I have never had a problem with the Kimber shucking one out and briskly putting another in.
Why would a person choose to go with a WSM? Maybe on a whim like me. Maybe you like short actions. Perhaps a person is looking for a beltless case which will get them in league with a 7mm Mag or a 300 Win Mag.
Suggestions: A push feed may have more luck with this round due to its shape. Slower powders like Magpro and RL 26 seem to help the WSM get where its designers intended. Check the capacity of the rifle, to be sure you don't end up with a 2+1 if you are not comfortable with that.