If you're asking these questions, then it is probably a situation where it won't really matter to you. Not trying to offend, but just from reading through your forum posts, I don't believe that you'll see much difference between safestrap ROMs and full custom kernel ROMs.
The answer as to why it's so hard: Samsung/Verizon/ATT work to prevent the bootloader from being unlocked. As such, the same as soft-modding a game console, an exploit needs to be found within the bootloader. This is the most dangerous area on a phone to mess around with, because it's very easy to permanently brick the phone when playing around with the bootloader. On the original droid that was the leading android development platform for a long time, the motto was, "If you can get to your bootloader, you're not perma-bricked." Screw up on the bootloader, and chances are, you have an expensive coaster.
Why is it so important? Locked bootloaders require signed kernels. Signing custom kernels is not going to be an option as it requires a proprietary encryption that is closed source. This can be thought of akin to MagicGate on the PS2 memory cards. (If you tried to create a device to read a PS2 memory card on any other device, it wouldn't work because of the MagicGate encryption.) It's NOT legal to distribute these encryption keys. As such, even if someone were to discover the key for it, if they distribute it to anyone, Samsung can, and as Sony proved, will go after them legally. Unlocked bootloaders can run any kernel that is made for that device. Unlocked bootloaders also allow flashing things such as a modem update without flashing an official update from the OEM. The primary reason, though, is the kernel. To answer your question from another thread, CyanogenMod requires a CM kernel. It isn't built for an OEM kernel.
Is anyone close to getting past it? Your guess is as good as anyone else's here. Probably not. Each thread that was started has been closed for various reasons.
Is anyone working on it? I believe there are a few devs working on it. If it happens, it'll be posted for sure.
question you didn't ask: Is it worth it? This is the important question you should be asking. Is it worth it to you? This isn't a question that can be answered uniformly across the android community. If you don't understand the benefits of a custom kernel, then you probably won't get much mileage out of an unlocked bootloader. If kexec is fully implemented into safestrap, then custom kernels can be executed giving nearly all the benefits of both worlds (safestrap is a wonderful recovery tool. If you brick w/ safestrap, it is PROBABLY bricked AFTER safestrap). The only real lack you have with kexec vs unlocked bootloader is the flashing of modem updates. This can be overcome by keeping your stock slot updated to the latest OTA (given that it's still rootable).