Reverb into high gain tends to terrible. Lower gain, or with almost entirely pedal gain in front of the reverb tends to work fine. Also depends on whether you want lo-fi or hi-fi reverb. I very strongly prefer most time domain effects in the loop or even better in post effects. Exceptions abound, of course. Univibe in front of amp sounds far smoother and more Hendrixy than applied in post...
A lot of modern amps have instrument level effects loops or switchable loop levels, which is perfect for experimenting with pedals. And there's some strange (and poorly documented by their makers in many cases) dirt boxes whose best tones are straight into the effects loop or power amp. MT-2 Metal Zone is a prime example. In front of amp, tends to sound muffled and be very hard to avoid the can of bees. Straight into power amp, it's massively easier to get useful sounds out of, as Ola Englund demonstrated, not that he's anywhere near the first to point that use out, it's been known on guitar forums for ages before youtubers started talking about it.
Most clean channel circuits in particular chop a lot of mids out, so unless the pedal is built to correct for a Fendery circuit, it's no surprise some pedals, especially ones designed to work as a preamp replacement or even some amp-in-a-box designs work better bypassing the preamp.
Non-preamp pedals do tend to work best in front of a preamp. And it's very rare for any gain source to work better in the loop (as opposed to instead of the preamp) than before the amp, unless it's being used to correct for poor output level of the loop, or for a loop/effect level mismatch.
