Historically speaking, he seems to have suffered more losses than victories and many of his legendary feats were actually fictional. I understand that he is worshipped as a deity more so for his loyalty than his might but I want to know why he was feared on the battlefield.
I'll start off with what Chen Shou wrote about Guan Yu in the San Guo Zhi:
Guan Yu and Zhang Fei can all be called a match for ten thousand enemies. They were like tigers among Liu Bei's subjects. Guan Yu had the style of a guoshi when he repaid the kindness of Cao Cao and Zhang Fei had the style of a guoshi when he released Yan Yan from prison out of righteousness. But Guan Yu was unrelenting and conceited while Zhang Fei was brutal and heartless, these shortcomings led to their downfalls. It was not something uncommon.
Guan Yu was renowned for his strength. As a general, he was not great, just decent, and far too arrogant. His only real shining moment was when he briefly served Cao Cao and fought at the Battle of Boma. The reason this is considered important was because he was one of the few generals who was recorded as personally killing an enemy general (Yan Liang) on the field of combat, rather than executing them afterwards. He would be rated as a good vanguard leader, but as a commander he was not that great, as his records speak for themselves.