There are very few patches that have been pushed upstream for this board to mainline Das U-Boot (yet). So while you would gain new features in U-Boot you would effectively need to repeat any work that starfive are currently doing to push patches upstream. And that is not simple (for most people).
As for U-Boot being relatively old, any needed core regression fixes would have been applied. If you think about it, the vast majority of commits to the mainline Das U-Boot are going to be for other boards that are not using a JH7110 SoC, almost all of which are not even RISC-V. So the other way to look at it is that for JH7110 this fork is the very latest bleeding edge U-Boot release (for this board). Anything critical that is needed for the JH7110 would be back ported. The reason to (temporarily) stick with one version of U-Boot would be exactly the same as for the kernel. It is the most efficient method in terms of time spent coding to create working patches which will eventually be accepted upstream.