I've never played GURPS (that I can remember - though the points-based character generation is dimly familiar ...), but I notice that GURPS hex maps don't make concessions to the hexagon shape and just superimpose the grid on a realistic map.
That's exactly the approach I've been taking with TFT - rectangular rooms where required, or natural cave shapes. I allow a part-hex to function as a whole hex for all purposes - so that you can get your back to the wall at any point, even if it runs through the middle of every second hex.
All this seems to work absolutely fine, not least in the weekend's island scenario; you can glimpse a poorly lit shot of a jungle clearing
here (the edge of the hex-map represented thick jungle in this instance).
Equally, 3D terrain seemed to work fine too; I just eyeballed however many hexes a bit of ruin covered and counted accordingly when players clambered up. And ditto with outsize monster bases (if the base is in a hex, the monster's in that hex).
But all the TFT materials I've seen adhere very strictly to megahex angles and dimensions. Am I missing something important in ignoring this, or is it just convention? Thanks in advance for enlightenment!