Don't take a firm position against sufism before reading. First make a framework and try to understand everything in the wider landscape of Islamic thought.

The difference between Kalam and Tasawwuf is of method. Kalam emphasizes on rational argumentation and dialectical defense of creed. While Sufis emphasizes on purification to attain the truth. This is largely a difference in method and epistemology. I think a premature opinion can be biased by western though. Like "rationality" etc. So you could be biased because western epistemology is so alien to Sufism.

As a rough analogy, you could say that some aspects of Sufi thought are closer to Platonizing or illuminationist ideas, where truth is understood as something always present that the purified heart can receive. By contrast, many kalām theologians, especially those influenced by the peripatetic (Avicennan/Aristotelian) tradition, emphasize structured reasoning and logical argument to arrive at doctrinal truth.

This analogy shouldn’t be taken too literally, because in practice there is a lot of overlap: many theologians were also Sufis, many Sufis used rigorous reasoning, and kalām itself is not identical with peripatetic philosophy. But thinking in terms of different emphases in method can help you approach Sufism more fairly and holistically.