It is my personal opinion, and entirely my own, that comparing players across eras that are radically different (Headley vs. Ponting, for instance) is a pointless exercise and however much one may try, consistency cannot be maintained.
It's better to go with gauging how a player stacks up against his peers. That's a fair indicator, IMHO. Jack Hobbs, for instance, was regarded as being quite ahead of his contemporaries with his sound technique and ability to play near-perfect even on dodgy tracks. In later eras, you'd find that a bunch of players are better than the others, but on very few occasions is an individual so far ahead of his peers.
And as most people agree, an ATG from a particular era would most likely remain an ATG if he's hypothetically transported to some other era. The basic raw materials to achieve true greatness haven't changed much in the game.
I agree with you mostly. However I am not sure that an ATG in one era would necessarily be an ATG in another era. He may be very good but not great.
And although I have done lots of stats analysis I understand totally that they are very limited and context specific and hence just cannot be used for comparison across eras. In case you want to attempt to use stats to compare across eras then you msut find a way to introduce the subjective aspects and quantify them.
Anyway, the more pertinent issue is when people start bigging up Bradman as the absolute greatest based on mythology. This thread itself has exposed that none of the "back in the day" brigadiers even have a decent estimate as to how many wet wicket innings were played and how poor Bradman was on them whereas 'lesser' ATGs like Headley, Hobbs, Sutcliffe, Hammond, etc have numbers thaty are bradmanesque in comparison to Bradman;s numbers on wet wickets.
Seeing the video clips of those guys I have no doubt in my mind that those fellows wouldn't have been in the caliber of Gavaskar, Chappell, Richards, Sachin, Inzi, Lara, Pawning, Miandad, Kallis, and Rahul. They might definitely make it to the second tier greats or decade greats like Zaheer, Vishy, Lax, Hayden, Lloyd, Chanders, etc