Many contemporary Acharyas say that Yajnas don't yield fruits overseas, or they say that Yajnas only yield fruits for the first generation of Indians living overseas, but that it's recommended to do Vedic yajnas in India.
The Manusmriti supports this notion:
But the region where the spotted deer roams by nature is to be known
as the ‘land fit for sacrificial acts’; beyond that is the ‘land of
the Mlecchas.’ (23)
The "region where the spotted deer roams" refers to the Indian subcontinent, and this is the deer that it's referring to.
Verses from other Dharma Shastras:
Vyāsa, 1.3.—‘The Vedic dharma can prevail only in that country
over which the black deer roams naturally.’
Samvarta, 4.—‘That country where the black deer constantly roams at
will is to be known as Dharma-deśa, where alone the duties of the
twice-born can be performed.’
The reason why it says to live in lands that have this type of deer is because the spotted deer's hide is a very important material used in many Yajnas, and since foreign lands don't have this type of deer, then you can't conduct Yajnas there because you would have no hides.
But what if you introduce this species of deer to foreign lands or export or ship spotted deer hides to foreign lands?
This is prohibited by stating that the spotted deer should roam naturally in those lands; or in other words, lands in which the spotted deer is indigenous.
But why should I live in India even if I don't plan on doing Yajnas with deerskin?
Because there are other reasons like Indian soil being more sanctified and pure on account of Rishis having lived there, there being lots of Dvijas, Kshatriyas to protect Yajnas, kings who can defend Dharma, Brahmanas who can teach Dharma and the Vedas, and the holy rivers like the Ganga, Yamuna, and the Kaveri, etc.
All men on the earth may learn their respective duties from the Brāhmaṇas born in these countries. (20)
Thus, the Manusmriti says that Dvijas should not even leave India:
The twice-born people should seek to resort to these countries; the
Śūdra may however, when distressed for a living, reside in any
And other Dharma Shastras even prescribe expiatory penances for travelling to foreign lands, implying that leaving India is actually a sin:
Baudhāyana, 1-30.—‘Āraṭṭa, Kāraskara, Puṇḍraka, Sauvīra, Baṅga,
Kaliṅga, Prāsūna,—if one goes to those countries, he should perform
the expiatory rite of either Punaḥstoma or Sarvapṛṣṭhā.
But I think there's good reason to think that these prohibitions of travel are purely for historic reasons, because people living in those lands were sinful, didn't follow Dharma, or were Mlecchas.
It's not recommended to live in Mleccha lands because the migrants and their descendants might actually become Mlecchas, as a Manusmriti commentator correctly points out:
The śūdra may go and live in another country, where there may be a
chance for him to acquire wealth. But even so he should never live in
a country where mlecchas form the majority of inhabitants; he should
betake himself to a land fit for sacrifices; because if he lived in a
country abounding in mlecchas it would be impossible for him to avoid
their contact, in the course of moving, sitting, eating and so fourth;
so that there would be the fear of his becoming a mleccha.
And we even see that 1st and 2nd generation Brahmins who migrate to Western countries become like Mlecchas by partaking in their culture of drinking, premarital sex, meat eating, marrying Mlecchas, etc.