I have the 14mm Samyang, but I can't really help with any of those questions, because I don't have any of those other lenses. Samyang is pretty sharp, has decent bokeh and performance. But it has it has many downsides.
a) Fixed lens hood. And its for full frame.
b) Not possible to use screw-on filters
c) Distance scales miscalibrated. Some say that all Samyang 14mm for Pentax have the distance scales of Canon. Not sure, but I know its hard to find the true infinity on my Samyang, and 1m is not actually 1m. Some also say the lens focuses differently depending on whether you start from infinity or from near.
d) No AF. MF is actually pretty hard with this lens because it is an UWA and everything "appears" in focus until you view the file at 100% when you notice you completely missed. Zone focusing might work, but you need reliable, calibrated distance scales for that. Basically, I would recommend you use a camera with big viewfinder with this lens. Maybe even use a special focus sceen. I often have a hard time with it on my K-01 (and focus peaking is useless with it)
e) Poor QC. You might get a bad copy with misaligned elements, and then you need to return it, get another one or get it repaired.. when you buy it, make sure it is equally sharp on all sides and has consistent CA.
On the other hand, it seems to be a tough lens, with good IQ once you get the hang of it and start using a tripod with high post-processing added sharpness. Here are some photos I took with the Samyang 14mm. They are pretty distinctive. And I pp them a lot, they are not out-of-camera jpegs. But they give you a taste of what is possible.
500px / 14mm Cathedral by Stolpulus II
500px / Golden path by Stolpulus II
500px / Snow prints by Stolpulus II
If I had the money laying around, I would probably go for the Pentax, either the 12-24mm or the well-liked 15mm. But at the price, the Samyang is a valid choice, too.