Size is occasionally an issue with heavily converted/outright stand in models. For example my chronos (formerly my talos, but it's build for close combat more than shooting so I switched it with the new rules) is based off the old metal dark elf hydra. It is no longer the right sort of size and shape for the new talos/chronos kit. Mostly because they CHANGED the kit and I can't be bothered trying to bend metal necks to a better match.
On the other hand, my custom converted beastmasters are now closer in size to the official model than they were previously. If someone seriously tells me I cannot use my units because they aren't the same ones GW manufactures - despite having lots of GW parts (I play in a GW store) - the clearly I don't want to play them because they're jerks and, in my experience, are the sort of unpleasant rules lawyers that ruin games.