The reason that you can criticize a Roman mosaic and not a Modern art mosaic is that the Romans set the patterns in their mosaics following a distinct set of rules. Sometimes this is referred to as Andamento or is classified as an ‘Opus’ (work) such an Opus Vermiticulum or Opus Classicum. For the sake of simplicity I use the term ‘The Rules’ when I refer to the way in which the Roman craftsmen worked.
It does seem that the Roman mosaicists used these Rules to ensure that no one area of a mosaic stood out as a result of the patterns in which the tesserae were laid out. They wanted everything to flow, for the viewer to see the patterns and not the tesserae. The floors may have had highly artistic figural works for example which form the centre of a floor but you don’t want any one section of the floor to draw in your eye. The Rules applied to every part of the mosaic, there may have been some differences in the quality of the work but these Rules were seen in all the mosaic.
Also, with stone you only have a limited palette of colours, you tend to work using 'shades' as opposed to very distinct colours. So you need to set them in such a way as to make the best use of what you do have.
There are eight Rules which can be seen in Roman mosaics but you also need to learn the exceptions to them and also how they can be affected due to the standard of an individual mosaicists work.
These 8 Rules are:
Borderline - Convergence - Keystone - Brickwall - Line end - Size - Spacing - Channelling.
Learn the Rules; apply them in your work and most importantly every time you see an original Roman mosaic look to find them.