apart from getting grants and other stuff, the biggest problem for a researcher is to win the rationale of his research in his brain. That is to believe in the core concept of the research question

Best,.

Mannan 

I think data collection, validation and reliability.

Best regards

Ammar

What are the most important challenges facing the researcher?

For me are both the data collection & writing up the discussion section.  Reason being data collection generally take the longest duration & require many follow-ups with respondents compares to my other research activities.  Discussion section of my manuscript also require much more attention & effort to complete.

For me, data collection and time.

time and comfort time to analysis material of research