Shaykh Maulana Usman Muneeb
Benefits and Issues:
➊ The meaning of "la'nat" (curse) is to supplicate regarding someone that Allah, the Exalted, distances them from His mercy. It is not befitting for a believer to make such a supplication for anyone, because a Muslim is merciful towards others and assists in goodness, and he desires for his Muslim brother what he desires for himself. And no person desires to be distanced from Allah's mercy for himself.
➋ On the Day of Resurrection, the people of faith will bear witness that the previous prophets and messengers conveyed Allah's message to their respective communities. Allah, the Exalted, will accept their testimony. This will be a great honor for the Ummah of Muhammad (sallallahu alayhi wa sallam), but the one who excessively curses and reviles will be deprived of this honor and will not be permitted to bear witness.
➌ On the Day of Resurrection, besides the prophets, the truthful (siddiqin), the Qur'an, the angels, etc., the believers will also intercede for their Muslim brothers who, due to some sin, have entered Hellfire—may Allah protect us from it. Allah, the Exalted, will accept their intercession and honor them. However, the one who, in the world, excessively cursed Muslims—even if he enters Paradise—will not be able to intercede for anyone. It will be as if he is made to realize: In the world, you used to supplicate for people to be distanced from Allah's mercy, so now you are not permitted to seek His mercy for anyone.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 316