For example, some researchers have conducted great researches, but they may find it quite difficult to explain the methodology or background info to the audience.
It depends on the exact context, but I would probably call them "bad/poor communicators", though something more tactful would be required if I had to say this to their face. The polite method would be to emphasise that someone else was a "better communicator", or "better at public speaking".
Thank you! Actually, I want to tell my audience that I am not quite good at explaining my research design at the beginning of my presentation to lower their expectation. So would it be ok to say things like "I am a poor communicator/explainer, but I'll do my best to show you the way i am going to conduct my research and the methodology behind"?– CarolineAug 12, 2019 at 7:07
Slight improvement would be to say "I'n not very good at explanations" or maybe "I'm not very good at public speaking" but ironically, if you use bad grammar at that point, you may well get them on your side a bit more than if you say it perfectly!– MikeBAug 12, 2019 at 9:31
Thanks again!! I take your point, but I have to add that having a good command of a certain language only doesn't ensure the ability to explain complicated things very well in the language. Sometimes it takes more than knowledge of grammar or vocabulary to make our opinions clear to the audience. Have a nice day,:-)– CarolineAug 13, 2019 at 7:22