I don't t know that it is so very difficult, except in the sense that like guitar, you can take it to a stratospheric level if you have the determination and the talent!
In koto, you set up the bridges for the key you are going to play in, and then depress the string on one side of the bridge with one hand to create the exact pitch you want, while plucking with big thimble-like picks on the other hand. In other words, the pitch is a bit easier, because you are using a type of fret.
Shamisen has no frets, and neck is very long - moreover, a lot of tunes are very fast, so you need to have a good ear and practice enough that your left-hand will hit the right spot! Big deal...
Different types of shamisen and lute in Japan use very different types of plectrum, some of which are absolutely enormous. These big plectra, as you may imagine, allow many different types of plucking, stroking, bashing and what not. That's somewhat true for guitar also, and just as in guitar, most players don't use every known technique.
One good thing about shamisen is that most types are portable - you pop the neck off the body, so it's much easier to carry around than a western stringed instrument.
There are 3 major types of shamisen, classified by neck size. The thinner ones are smaller all over, cheaper, and easier to play (they are mostly used with fingerpicks or small plectra, as accompaniments). The Tsugaru shamisen is the biggest, the most modern, and because it makes more noise, is used more as a solo instrument.
In particular, a few years back the guvmint decided to introduce Japanese instruments in school music education. Suddenly all talk of "nothing but silk" for strings and "nothing but catskin" for the instrument tops disappeared, and affordable, durable, entry-level instruments have appeared. Yay! Silk strings have a lovely sound, but they are absolutely NOT durable, they are a pain in wet weather, and they go out of tune every time you look at them. Koto has the same problem.
Intro to Tsugaru Shamisen