In 1797, R. Abraham of Kalisk, from the Holy Land, wrote a poignant letter attacking the Tanya. His concern was the explicit nature in which R. Shneur Zalman endeavors to explicate the message of the Besht and Maggid via Lurianic concepts.
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R. Shneur Zalman wrote Tanya as a systematic guide. His intention was to uplift the masses and allow them to genuinely connect to the higher chain of mystical literature. However, R. Abraham felt that the great majority of Hasidic constituents had no place on that path. Rather, they were only meant to absorb the simple teachings that were placed before them. He did not trust the Hasidim; he did not think that they had the proper tools to understand the true teachings of the Besht and the Maggid.
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R. Abraham of Kalisk’s point of contention, the manner in which R. Shneur Zalman explained the Hasidic doctrine, is exactly what made Tanya provocative and unique. R. Shneur created an organized and intellectual manual for his disciples. His goal was to use kabbalistic language in order to rationally elucidate the teachings of the Besht and Maggid. The Tanya employs an intellectual perspective to Hasidism; for this is precisely the meaning of Habad—Hokhmah (wisdom), Binah (understanding) and Da’at (knowledge). R. Abraham felt that empowering the Hasid himself to understand, rationally, the depths of the teachings takes away from the centrality of the Hasidic leader. The leader of a Hasidic group, referred to as the Zaddik, is not a mere communal head, rather he expresses cosmological and spiritual powers. This is perhaps the greatest innovation of Hasidism. R. Shneur Zalman changed this socio-religious dependence that the Hasid maintained towards the Zaddik, since he no longer had to rely on his faith of the Zaddik. Now, the Hasid had direct access to the esoteric teachings.
R. Shneur Zalman changed the leader/disciple relationship by establishing local guides, students of high intellectual pedigree, who assisted others to understand the meaning of Tanya. This was a further critique of R. Abraham of Kalisk, since Hasidic teachings in his mind were not intellectual teachings that could merely be explained. Rather, the illumination of esoteric Hasidic teachings required charisma. He maintained that they were meant to only be given over by a highly elevated and spiritual person— the Zaddikhimself.

Ben Vago, “Head to Head: Power Struggles in the Creation and Formation of the Habad Movement” Milin Havivin 4 (2010), 136-137.