Significantly smaller scale for some gangs.
Escher are the best fit as virtually everything on the stock metal models can be used, even things like the power sword on one of the leaders. Escher are also the best fit style wise with the new plastics although the new models are quite a bit taller and broader especially with their high heels.
Van Saar are also ok, although the decision to give las versions of autoguns and shotguns will make their models look a bit odd as they are obviously projectile weapons and not las ones and their overall gang aesthetic looks completely different, so the new models aren't really compatible. But again virtually everything they have on the stock metals is usuable.
The other gangs are missing a lot of common things, especially the Goliath's who lack a lot of their basic gear from the original models. The size difference of the Orlocks and Goliaths is also very noticeable, they really don't look good together side by side. Cawdor are even worse with how primitive they are in the new versions and as with most things these days the less said about Delaque the better.
The biggest issue you will face is the lack of specialist champs and everything from the 'House of...' books onwards. If you were to stick to the gang lists from before that you would probably have an easier time adapting your old stuff, although the other players may not want to do that. And the scale issues will make it hard to add those things to your classic gangs without a lot of converting.
Give it a try, but be prepared to either do a lot of converting, a lot of house ruling or accept that you are going to be very underpowered Vs the latest lists.