A properly designed loop is a properly designed loop, whether or not it is tube doesn't matter. Buffered means the loop has a low impedance output so that putting effects in the chain won't load down the output and a gain stage to bring the signal back up for the power amp.
Un-buffered means you are just interrupting the signal path between the power amp and preamp with no low impedance drive and recovery stages.
Many tube amps have buffered loops but he output is +4 dB line level, and most pedals want to see -10 to -20dB guitar level. In this case the amp loop overloads the pedal causing massive tone sucking. Some of the newer pedals can take the higher signal but many can't.
Good loops have either a +4/-10dB level switch or adjustable send and return gains to compensate for pedals and or rack gear.
The metro loops are popular because they don't require a DC regulated power supply like OPAMP loops and draw very little current, so the can be tapped off the High voltage B+ without drawing down the B+ too much. And no need to ad a tube, etc.
The only absurd thing about loops, is people saying they only want a tube loop to preserve the tone. Because when it comes right down to it. Most effect pedals are solid state and the signal passes through OPAMPS anyway that are in the pedals.
