A few weeks ago on Fury FC there was a +900 underdog (Koery Windham), one of the most decorated high school wrestlers in the history of New Mexico, that also had better striking and much better conditioning than his opponent. His opponent (Lester Batres Jr) had such bad conditioning that he was gassing in the middle of some of his amateur fights where the rounds are only three minutes long. Given all of these advantages I felt Windham should be a favorite in the neighborhood of 55-60%.
Windham despite holding all of these advantages lost 29-28 because his opponent wanted to win more. His opponent was willing to totally exhaust himself and didn't care how he won, just as long as he won, and in the end he did win.
These ^ are a dime in a dozen. If I bet a line every time I believed there was value on it I would go broke very quickly. Value often does not get there and good bets lose all the time which is why you need to carefully choose your spots and put your money in a strong position to win.
Hopefully you understand now why to be a long-term winner you must take a disciplined approach to betting.