There's evidence suggesting that "the A118G polymorphism in the opioid receptor mu 1 gene (OPRM1) is associated with decreased opioid receptor availability, altered emotion, and increased pain". This genetic variation leads to increased reward from opioid drugs as they help compensate for gaps in the endogenous endorphine system. Of course, that's definitely not the sole factor, the environment and the mental health complications it may cause are very important as well.

Do you think that's true? Do you know many people that just can't get significant euphoria from opioids no matter what they do and for them this class of drug is just "meh"?

It makes sense because many don't find alcohol and benzos highly enjoyable either whereas another subset of the population tends to get horribly addicted to those.