do you have some hard facts for that?
Yes, I hear this noise with my own ears. One time one of these amateur with Meanwell PSU got linear one for a short period of time. And when he tried to work with linear PSU, I notice very well that background noise disappears. The test was very impressive because the difference was just amazing. The sound of his station with linear PSU became crystal clear. With Meanwell PSU it was unpleasantly noisy.
Also he reported that he got much less noise floor with linear PSU and can listen much more weak stations, which is under the noise with Meanwell PSU. Before that case he advertised Meanwell PSU for TRX, but after that he is not so happy with it anymore
I know that Meanwell is often first choice in the ham domain
Yes, exactly. As I said above, many ham amateurs in my location using Meanwell PSU. But it doesn't means that this is better choice. This is just a cheap, lightweight and compact compromise. But it affects your receiver sensitivity and add noise on TX.
Some time ago I also tried to use Chinese SMPS for my QRP TRX. Cheap, small and nice... But later when I compared it with linear one, I never use SMPS for radio anymore. Because SMPS produce a lot of noise.