@Malavika Ah yes, this is comparable to Sudhamantram and Bhavayami (though I think the former is actually even more difficult, and I'll explain why). The difficulty comes from grahabhedam, which is what all these songs are built on

Of course, nowadays this has become a common thing, and many Carnatic and Hindustani singers do it during ragalapana or swaraprastara — notably, Balamuralikrishna who used to do this even long ago when it wasn't so popular.

And you must listen to his Thayaragamalika tillana, which is basically in Kalyani, if you look only at the scale, but is actually a ragamalika because it goes into other ragams (Shankarabharanam, Mohanam, Hindolam, Darbari, as far as I can remember) by grahabhedam.

And there's also his RTP with pallavi Sarigamapadani Padeda, in Natabhairavi, in which during swaraprastara he does grahabhedam and sings all the other possible ragas, the same scales that Shadaj Ne Paya explores (except, starting from Shankarabharanam instead of Natabhairavi)

As to why I consider Sudhamantram more difficult… Normally the grahabhedam is done for a new line, in a composition, or in swaraprastara, again you get some gap to establish the new aadharashadjam, where you can hold it for some time and then shift to it to get the new raga. But in Sudhamantram, the shifts are done in a very short span of time, or in one line of the song itself

It's there in the first alapana itself (which, when you're hearing it for the first time, sounds like the singer has made a mistake)

Also, it is different from the usual grahabhedam because the shift is not to a note that's present in the raga. Instead, what he does is shift the shadjam one swara down (i.e., one semitone down), and keep the other notes in place, which gives a new raga that actually cannot be obtained by pure grahabhedam

And this is done in an impossibly fast manner

Bhavayami has the more conventional grahabhedam (and it also uses different ragas in the usual ragamalika form, not via grahabhedam), so it's much easier to sing if your voice is trained enough (of course there are physical difficulties in terms of speed and range, which is a completely different matter, and can even apply to say, Harimulraliravam or Pramadavanam).

But even in Bhavayami, he does employ some fast-paced grahabhedam during the swaraprastara, which is still relatively easier compared to Sudhamantram.