Last year, San Fran drafted Isaac Guerendo in the fourth round. He put up good numbers in a limited role (84-420). Before that, they drafted Tyrion Davis-Price in the third round in 2022. He's had a total of 43 carries. They drafted Trey Sermon in the third round the previous year, and he's had a total of 134 carries. Elijah Mitchell was drafted in the sixth that same year and has had 327 carries. Joe Williams was taken in the fourth round in 2017 and did absolutely nothing. Same with Kelvin Taylor taken in the sixth round in 2016. Counting Jordan James, taken in the fifth round this year, that's their RBs drafted in the last 10 years. (I don't have a list of their UDFAs during that time, but guaranteed there are a lot more misses than hits.)
Not sure what your idea of a hit is, but none of those six backs (not counting James) has averaged more carries in their careers than Guerendo's 84 last year. Is that what you would consider a success for Blue or Mafah?
Guerendo did average 5 yards per carry in his limited role and Mitchell 4.6, so there is that, but Davis-Price averaged 2.9, Sermon averaged 3.7, Williams nothing and Taylor nothing. So at best, the Niners hit on 2 of 6.