One of the issues with the One Spot is that it is a switching power supply (also called a Switched Mode Power Supply - SMPS), instead of a linear power supply like most of the wall warts we're used to using for guitar pedals.
Switching power supplies do have some good points. They can often be made smaller, produce less heat, and waste less electricity than the legacy-type linear power supplies. But, the action of the switching transistors in them can create electrical noise.
Here's a white paper from Strymon where they discuss their switching power supply, and what they've done to make it noise-free.
https://www.strymon.net/effects-pedal-power-supplies-white-paper/
Down in the section about switched-mode power supplies, they highlight the noise issues that can occur with them.
Here is one interesting statement from the white paper:
"The added efficiency of these supplies comes at a price. Compared to linear power supplies, SMPS are more complicated to design, especially for ultra-low-noise applications like high dynamic range audio processors, since they have the potential to generate significant amounts of noise."
I've confirmed this on my own, where I've plugged a One Spot into a pedal and gotten a hum, but not with a Boss power supply. The One Spot is a switched power supply; the Boss is linear.
Now, of course, there are pedals that probably work fine with a One Spot without noise. I'm not flat-out condemning them. If someone is using a One Spot without issues, I won't argue with success! But, if you have an odd hum that you can't eliminate, consider the power supply for your pedals. The "dirty power" may not be coming from the wall...it may be coming from the power supply, itself.