Opponents of immigration, including nationalists and theocrats, often speak of the benefits of cultural homogeneity and social cohesion. They have gone into great detail on those benefits, and I agree with much of what they have to say about them, so I will not discuss that here.

Here, I observe and detail certain drawbacks from social cohesion and a shared culture, or at least reasons that the benefits of cohesion cited are not as good as they seem.

• Systems based on social cohesion are often not scalable or not generalizable. They often involve procedures or practices that are based on everyone in a group knowing each other and having a reasonable idea whether someone is lying, what their issues are, and so on. They also sometimes involve using benefits created by the broader non-homogeneous society. Here’s one example: Amish don’t sue doctors for malpractice; this is enforced by social pressure and by their version of health insurance being provided by their church. This allows them to get better prices for medical care, because the costs of lawsuits (valid and otherwise) don’t have to be priced in. But if everyone was prevented from suing for malpractice in similar fashion, doctors would have no incentive to avoid making harmful mistakes. In a real sense, they are free-riding on everyone else’s willingness to sue for malpractice.
• Costs apparently eliminated or prevented by social cohesion are often just taken off the books or made not legible. If shoplifters are tackled by random citizens in a socially cohesive society who otherwise escape and thereby impose costs on retailers and customers, that can be considered a benefit. But does this properly account for the costs to those random citizens of risking violent harm without financial compensation (or, at least, none guaranteed), or to the shoplifters themselves who may be killed or sustain injuries requiring expensive medical care as a result of being arrested by untrained people?
• Social cohesion encourages conformism; melting pots are better at having the Hegelian dialectic between cultural practices and values not happen through violence. This is important because violence is very lossy, and (contrary to Hegel’s thoughts) selects for the wrong things: militarism over seeing the military as just another public service, zero-sum thinking over positive-sum thinking, and strong in-group preference over universal human rights. This is why, for example, you had Prussian (and to a lesser extent French) militarism leading to two world wars even after war against peers had become maladaptive. Even peaceful competition between nation-states with different values can be wasteful and select for the wrong things: economic power may derive from skill at currency, diplomacy, and tariff manipulation, including proxies for ability to credibly threaten violence, rather than optimally satisfying tangible and intangible human needs and desires. If everyone is in the same market and society and has to compete non-violently, it is easier to select for cultural ideas and practices which make one happy rather than those which make one good at collective violence.
• Stupid laws that are not in practice enforced on in-groups or groups with political power (“smoking marijuana while white”) can persist much longer and cause much more harm to society than stupid laws equitably enforced, because the politically effective negative feedback mechanism is eliminated. Social cohesion among those in-groups reduces enforcers’ willingness to enforce such laws on in-group members.
• Social cohesion increases social pressure to redistribute wealth, which in turn disincentivizes creating it (this is specifically identified by Jared Diamond as a reason for lack of development in New Guinea, and Africa has similar problems). A homogenous working class can more easily coordinate to extract from business owners through electoral politics or labor organizing in ways that are harmful to efficiency and individual liberty, and thus to long-term economic growth.
• In a system of decentralized political power, social cohesion encourages selfish/rent-seeking behavior by social groups in control of political subunits to the detriment of broader society, in ways that they cannot coordinate with less social cohesion. NIMBYs preventing housing development and thereby massively increasing housing costs in a regressive way is an excellent example. In other words, cohesive groups can coordinate to work against the interests of a broader society, or those of humanity, in negative-sum ways that are harder for more diverse ones.