Coming from someone who worked in a company that received Softbank investment, it's a huge hassle. The ONLY reason to take Softbank money is to get a huge sum of money at a huge valuation and reduce your "cost" of said money. But for me personally, the costs aren't worth it.
First, there's the inflated valuation. It's a great MO for Softbank - they get you by stroking your ego and flashing big (exaggerated) valuations. But does anyone ever think about the next round? No one else after is going to invest at that valuation + extra, unless you really knocked it out of the park, so you are stuck with them for future rounds. Or IPO (Hello Uber). Startups are hard - why are you making it more difficult for yourself to succeed?
Second, the ongoing effort. You don't have one or two guys at a VC firm you are working with, you have a bureaucracy you are working with, in a completely opposite timezone, with a very different working culture.
Maybe it makes sense for a company who is on their last round of funding before IPO, just needs one last cash push to get everything there, and can pay a bunch of people to deal with the BS. But that's not most startups.