Agree, there are good and not-so-good.
I'm paranoid enough now that on a crate I'd want to open it up & double check, and I'd want to run it on a test stand before plugging it in.
But much of what I see advertised seems a pretty good deal. Time is money in some situations...
About 30 years a go a noted & well-known SoCal engine builder with Hemi experience from his days in SS driving & tuning for Landy, rebuilt my tired Hemi.
Ran great - still does - though I had to get the carbs sorted out.
10 years after that, he went through my drag-race SB, and it had problems right away, and I took it back to him.
Those flaws went uncorrected & turned fatal on its 3rd pass, only saved the intake & valve covers.
$10,000 out the window, teardown showed horrible stuff, valve reliefs milled through the pistons, oiling & fastener & torque mistakes, I could go on.