see, "difficult" or "tough"..these things are based on perceptions.
I mean to say, some may find it tough & some may find it easy.
Now its very tough to decide whats tough & whats easy in a general perspective.
Regarding BE, cant compare coz it has a tough entrance test, AIEEE. not all get through and those who do, dont find it tough later in college.
IAS: One day game like CAT. severe concepts, speed & control over mind is reqd. then again, its an Exam. CA is a qualification involving 3 levels of exams.
CS: no doubt its' easier than CA.
So its difficult to compare & judge.