It's beyond dispute that Ishy had his earnings for the Shadow during the time, when he was free from the Bore. Nevertheless, he wasn't able to find the child Dragon Reborn in time, later suffered three big defeats against an untrained Rand & was killed, apparently failed with most of his DF plots and lost control of Fain without achieving his death yet.

As the Nae'blis Moridin has still not managed to firmly control the other Forsaken (e.g. shown by "Sammael's" Trolloc army in KoD). Moridin usually needs SH/the DO himself to reprehend the other Forsaken (e.g. by rape or mindtraps). Moridin failed to stop the using of the Bowl of the Winds to repair the weather. He also couldn't stop the Cleansing (I think the Nae'blis should take responsibility for his fellow Forsaken, who he couldn't even help). He helped Rand to survive in the end of ACoS and kill Sammael (BS said that the DO would like to have Sammael again). During the same event Moridin became by accident linked to Rand, which in the end helped Rand to kill Semirhage and Graendal, although it's correct that the DO almost won since Rand considered to destroy the world. Finally, Moridin's orders to kill the other ta'veren or find the last Seals remained unfulfilled yet.

On the other hand, Moridin seems to be able to gather the Trollocs in the Blight and win in the meantime most sha'rah games against himself, if he isn't giving Rand tips how to defeat the Shadow..

Sure, Moridin is closest to the DO's attitude towards the total destruction of everything and did help the Shadow in the past, but is this enough to still justify his current status as Nae'blis? Shouldn't Demandred get a chance to prove that he would be a better Nae'blis (he didn't seem to be such a bad leader in LoC)?