I agree..Toughness, in technical sense, is a very relative concept. But in that also, there are some aspects which are really prominent than others and are fairly the same level of competence for majority of singers. For example, Aditi may have classical expertise; but what I've heard so far from her still makes me feel that a classical song with fast 'taans' will be still be a tough thing for her. But yes, if she does lot of pratising, she can do it.
Do u think Abhilasha would not hv practised the song 'Tu chanda Main chandni' a minimum of hundred times (if not so she would hv been singing the song since a few yrs in other competitions be it in her school,college,etc)
So, leaving apart some typical singers who are good in one genre, a tough song should be tough for the majority of singers. It may be classical/semiclassical number or a rock/disco number, some common factors can be - a song with varying tals, a song which spans over a wide range(scale), a song with lot of harqat in a limited range of surs which vary slightly everytime n consequently, a song with up-down notes which continue for a long time, a song with a stretchy note for a really long span of time which requires a good control over 'Saans'(what Anup Jalota/Chanchal did very efficiently), etc.