I wont say it is difficult, But a longer process, like if you are making tofu/paneer sabji, you are also making the tofu yourself.
I that case posto is little more difficult, because marking the paste is the art, it should not be too powdery which will make bitter, and if it is less done it won't be smooth while eating, one can use readymade paste which are available in the market, It is OK but not as good as diy. so it takes a lot of tries to get there...
Also please tell your favorite veggies, so that some more Bengali food...