So if Al Hilal was willing to take a risk, and paid him a lot of money while he was missing games due to injury, why do you think that Miami won't do that? They could take that chance and hope for the best as well. Neymar could be injured, but Miami has to take that risk if they want to get better, and if they do, then they will see if it was a good move or a terrible financial decision.
Miami might not do it because Al Hilal took the risk, gathered the data about Neymar painfully and had to realize that he wasn't worth a single penny of the salary he was making. Miami would only sign a contract with Neymar under very different terms, by far not as much money as Neymar used to make and there is one more reason why Miami might have an interest: it is the global presence that Neymar has and he would be bringing in a lot of new fans from all over the world. It would be more of a marketing move than the consideration that Neymar could help the team to improve over time. Neymar is done as a football player. Yes he will still score a goal every now and then and it will be fun to watch him from to time, but his contribution to the game overall will be limited.