The official GWR reason was that any failure in the gear rendered the whole gear completely inoperable, and made it impossible to limp home on one side. The 4 cylinder locomotives were provided with an special link which could be used to hold the gear up out of use in case of a failure, whereas other lines simply provided instruction on how to achieve the same effect with blocks of wood and tying things up. Gibson, characteristically, has a conspiracy/cockup tale about how the gear was very difficult to set valve timing on and it would take days to do. I've also heard it said that the valve events weren't great because of the angularity problems alluded to above. All three could be true of course. The GWR gear was quite different to Deeley's design, which involved different length rods, so as cross connnected gears had been done before there is at least a doubt whether Deeley's patent would have been valid.