I don't think either team underestimated, Phillies had Long watch film on Marsh, and it seems he saw a flaw he thought he could correct, O'Hoppe has shown a lot of potential, but he's still only in AA, which leaves a big jump to the Show.
In both cases, there's a lot of risk and a lot of upside. O'Hoppe is an overachiever, but where's the ceiling?
Marsh has been an underachiever, but flashed the ability to be a plus defensive CF with .750+ OPS.
Marsh was rushed, only 1330 PA before he was promoted,
wRC+: A+ 102 at 20 years old, AA 137 at 21, AAA 108 at 23 (110 PA) before he was promoted.
Missed a full season at 22. Should have spent that season in AAA ball.
BB rates 12.2%, 11.4%, 14.5%, in those three seasons, so you know he's not a free swinger.
He has a chance to be a leadoff hitter with a high OBP with some pop to go with his defense.
At 6'4 215 there's probably significant untapped power.