If you are buying directly from Apple on full payment then there will be will be no issues. Carrier phones (T-mobile, Verizon etc.) do have some signal issues here in Pakistan.
It is not like that. There are two ways to purchase phone there. 1- You buy directly from Apple (the unlocked versions) and you can use them anywhere around the world. 2- You buy from carrier company (T-Mobile, Verizon, O2, Vodafone etc.), it is like buying a phone on installments and these are carrier locked phones. You'll have to get them unlocked before using any other network sim.