We need to keep in mind that Willie Mays is a special case because, in real life, he produced huge stats in the minors and had a great rookie season in 1951. But then he saw limited action in 1952 and missed an entire season in 1953 due to military service. Then he returned in 1954 and was instantly one of the best players in MLB. This means it is absolutely
crucial when OOTP imports him.
If Andrej72 had started his game in 1951 or 1952, Willie Mays would already have really good current ratings in addition to his great potential. If you import him in 1954, then he's going to have some of the best ratings in MLB. In those cases, he will probably reach his full potential or he will already be at that level.
However, with the 1953 import, Willie Mays is given current ratings based on his much weaker 1952 stats, during limited playing time. That gives him really poor ratings and a much lower starting point at the age of 22. His ratings are also adjusted and tend to be worse due to his low number of AB in 1952. Having low ratings at age 22 increases the risk that he will never develop to his full potential because the development engine often starts to slow down and reduce player development and potential as they reach their mid-20s. That is exactly what happened to Mays in a test sim that I just ran.
I just ran a simulation starting in 1953 and using the same settings that Andrej72 is using in his game. Willie Mays improved during 1953 and 1954, but as he got a bit older and reached the age of 25, the development engine reduced his batting potential ratings. Now his potential ratings are average at best, and it will be impossible for him to reach his real-life talent level. This resulted in him being stuck in the minors and unable to perform well during his brief MLB opportunities with the Red Sox, Cardinals and Phillies. But then he was claimed on waivers by the Washington Senators and finally got a chance to be a starter for a full season. Just imagine Willie Mays on waivers!

In 498 AB with the Senaors in 1958, he hit .251 with 7 HR and 76 RBI, but based on his current ratings and potential, and now that he is 27 years old, he probably won't be able to do much better.
With the development engine, I have found that players with very high potential often develop into stars, as you would expect, and often they become good or very good players. But, yes, there is a chance that they will never make it out of the minors and be a complete failure. Their initial current ratings and their age also seem to be huge factors. If a player is already pretty good when he's created, and he has really high potential, it takes a much shorter time and distance to get from his current to his potential ratings. But if he's already 22 years old or is 18 and starting with really low ratings, then it's a very different situation, and it seems that there is much higher risk that he will not reach his potential.