From my own limited experience, I can say that magnetic loops work and are small. They are not ideal where frequent changes of frequency are required (if efficient they are very sharp) but they are ideal for modes like FT8 or PSK where most of the time you are within a few kilohertz.
With high power, often vacuum sealed capacitors are needed. With QRP, ordinary capacitors are fine. Even though the voltages at the high impedance points are high, they are nothing like the kVs seen with higher power.
I suspect loops are down on full-sized antennas, but they work pretty well. Being narrow-band probably helps keep QRN down too. In the absence of external antennas they can be used in the shack. If making the loop yourself use the thickest material you can for the loop inductor: I use central heating copper pipe, but I suspect thick coax would be fine. You don't need much.
I see Southgate News has a piece on a new commercially available loop although it is quite easy to make your own.
See also https://sites.google.com/site/g3xbmqrp3/antennas/magloop
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