The major problem with Scout is its opportunity cost.
I will routinely grab Scouts off of otherwise dead Iw gains in the mid-late game. Late game Scout is pretty good at increasing reliability - have a drawing village + Scout? Then your search space for your draw from 4 to 7 to line up the draw, deck ordering is pretty useful for those turns when you engine is starting to falter. Reordering the deck top is not a trivial ability - Golem (both so you can ensure that you draw Golems into hand and not skip them and to order actions so you hit the non-terminals first), Lookout (so you can efficiently trash & discard), Nv (use the mat as pseudo-trashing), and some other cards all care a good bit about what is on deck top.
Why is this rarely worth buying then? Because virtually always there is something else that gives you more reliability that is better up until that something runs out - e.g. more draw, or more villages. However if you've been using Iw to grab villages and silvers with Hunting Grounds for draw, once the villages are gone (and maybe slightly before) Scout is decent freebie. Sure you might have a spare +buy and $4 coin late game without components/silver you need to buy ... but when Scout starts to be useful you also need to consider Estate. Cards that can gain actions only (like Uni or Iw) may well say Scout is worth gaining just because Scout is marginally useful and it doesn't compete with estate.
Certainly I cannot fathom an equal opportunity cost Scout (e.g. from a Uni) being worse that a Pearl diver > 99% of the time. Do I win games off Scout? Ehh hard to tell. Certainly I think I have an edge when I mix a Scout into an Apothecary stack (very often making Scout a Lab or better, often letting me chain an additional Apoth that was buried 2-4 cards down on the deck) or when I add Scouts to a Pool deck (particularly if there is nothing cantrip at the opportunity cost). The marginal gain there is, at best, something like I might win 15% more of those games than I otherwise would (and this number will get smaller the better I become as a player), so most of the improved performance hard to pick out of the noise.
Scout does something useful that is often worth -1 card ... just not often worth it when you could buy a tiebreaker estate, silver, or an engine component instead.