Principles, nearly by definition, are rules that when they are adhered to, lead to good results. Group loyalty, then – loyalty to specific preferred people over principle – necessarily produces worse results. The two are not the same, so to maintain loyalty to a group, one must depart from principle.
Countries differ on the ubiquity of group loyalty over principle. Prosperous countries tend to be more loyal to principle; shittier countries tend to have more loyalty to groups. It takes a prosperous environment to be able to afford to be loyal to principle, but loyalty to principle also makes a prosperous country. In comparison, people in shitty places feel like they need to be loyal to their families and local groups, but it's also the group loyalty – and lack of principle – that makes those places shitty.
You can find shitty subcultures where group loyalty dominates even in prosperous countries. Compare inner city ghettoes with the "snitches get stitches" mentality. Or the lifestyles of gypsies in Europe, whose loyalty is only to their group. Poor people often have high loyalty to family – because they must, their family is struggling – and this is what helps keep them poor.
To the extent that group loyalty exists among the elite – the rich, the police – it serves to bring down the rest of the country. Group loyalty among the rich results in an economy that benefits them, instead of most other people. Group loyalty among the police results in them mistreating, often killing people, and finding no wrongdoing among themselves.
It needs to be accepted as an ethical standard that people need to be loyal to principles. Loyalty to a group, ignoring principles, needs to be viewed as a form of corruption.