Which tones are hardest, why, and where to start.
Yoruba is itself tonal. A comparative study found Thai speakers produce Mandarin tones better than Yoruba speakers, attributed to Thai's larger tonal inventory being more similar to Mandarin; the two groups share similar error patterns. No specific error direction is given.
Source: Gandour & Harshman (1978)（基础性研究，非普通话） | 泰语-约鲁巴语比较研究（未取得原文）
Group averages tell you where to look, not what you actually do. Two speakers of the same first language can differ a lot. The only way to know is to record yourself and compare.
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