How are people struggling to understand how useful this stand is to an assassin or calling it a bad stand?
If I'm reading this right, it says he controls the bullet speed, meaning he doesn't just slow them down, he speeds them back up as well. Take the end of this chapter with the elevator. You fire off a bunch of bullets aimed at where you plan a target to be, slow them down to practically nothing so they stay in place like a trap, leave the scene so they don't suspect you, wait for the target to get in position, then immediately speed the bullets back up to regular speed, let them hit the target just enough to dig in, then slow them down again to maximise damage. Also for all we know theres nothing saying he can only do it to bullets, that's just all we've been shown. Even if it only applies to bullets, and the bullets are part of his stand Emperor style, that just makes it more useful, since any non-stand user target is completely incapable of surviving, and since his bullets seem to melt into nothing afterwards like we see earlier in the chapter, even against stand users he leaves literally no evidence.
TLDR: Like someone else said, it's like a bullet version of Kraftwerk that can be toggled on and off and used at range without physical contact.