Ride quality is one of the Electostars' weak points. Try using the toilet on a Brighton line train at speed.Ride quality is very good overall on the 'Stars whilst Desiro ride quality is perfect on good track, encounter any bad track or some bad points and you get thrown from side to side, the ride on the 'Stars is much more conventional and overall it is is better.
They are not as smooth riding as the CIGs, and that was obvious when by types were in service together. I believe the explanation is quite simple and the solution equally simple, in principle. Mark 1 stock vehicles had couplings that put them in compression. Adjacent gangway face plates acted as a type of shock absorber to provide inter-vehicular damping. This was realised when mark 4 stock was introduced and suffered from poor ride quality originally. It is a good example of if something is not bust don't try to fix it. The mark 1 gangway was an inexpensive and simple design that did its job well.
The mark 4 problem was solved by fitting "buffers". Trains such as the Norwegian EMUs have long diagonal shock absorbers fitted between adjacent vehicles, which provide some damping, but it is something else that has to be taken off and put back when vehicles are joined and separated, and something else to go wrong.