I no Lubavitcher but I understand the point of view of the 6th Rebbe about Daf Yomi. I heard from elderly Chabad Chasidim who actually knew him that he did not advocate it because he felt that the mass learning of Daf Yomi would replace the type of learning that had been the common approach for hundreds of years in Eastern Europe. There were different groups for people of different abilities and inclination. There was a chevra shas, a chevra mishnoyis, a chevra ein yaakov, a chevra halacha etc. etc. He was uncomfortable with the promotion of the mass learning of Daf Yomi which (would have) replace(d) those groups. On the flip side he did not oppose Daf Yomi . Had Daf Yomi been another group associated with Chevra Shas he would have supported it. He never said his concerns mentioned above publicly because then people would (incorrectly) say “The Lubavitcher Rebbe opposed Daf Yomi”
Even though his concerns may have been valid in the 1940s today the chevra shas, chevra mishnoyis, chevra ein yaakov, chevra halach etc. are anyway long gone. If someone feels he will accomplish more by learning something other than Daf Yomi, then he should go ahead and do so. But the intrinsic concern the 6th Rebbe had is no loner relevant
As far as the 7th Rebbe and his approach goes … I’ll repeat something else my friend father was once told by the 6th Rebbe. “Chasidus can bring a person to very high madrogos but only if he keeps himself tied down to this world by learning gemora, rashi and tosofos” Enough said