Just that, changes to one will not be going into the other, separate development paths so there will be no reason for functions in MAX to work the same way as GGC or vice versa.
Cine|Guru was written back in the days where MAX was supposedly going to be backwardly compatible, i.e. you could import levels from GGC and expect them to work.
That is no longer the case so developers (like me) will have to evaluate on a case by case basis whether our products will be able to support both engines or whether we will need to create separate versions for MAX, if that is the case then it will mean another period of development and testing and probably extra cost to our customers, either that or if it is not deemed worthwhile possibly dropping products altogether.
What makes the whole thing so much more difficult is not knowing which functionality that was relied on in GGC is no longer available in MAX without a ton of trial and error, global.lua provides minimal documentation of new functions added to MAX but nowhere is it documented what existing functions are either not supported anymore or which work differently than they did (or indeed which ones are just broken by bugs introduced during MAX development).
Been there, done that, got all the T-Shirts!