My recollection from an interview somewhere was that Beltran seemed to resent how the writers gave Picardo so many good storylines and things to do.
If I got this right, Beltran was told that "the Doctor" was getting good material because Picardo would come up with ideas himself and pitch them all the time. And Beltran complained about it, essentially saying "that's not my job" and you should be coming up with better things for me to do.
Sour grapes. A tv show is a dynamic thing, & ultimately, you get out of it what you put into it, especially when it's kind of a soap opera format set to sci-fi
McNeill's Tom Paris had no more a gimmick than Beltran or Wang, & what HE got was a relationship, which is what O'Brien got too
& usually those go to the characters that are most likable. Had Beltran & Wang done more to present themselves as being more interesting, then they'd maybe have been more likable or at least more engaging
Then you get better stuff to play. It's a two way street. You can't blame it all on the writers. Most of the time they're just writing for the people up on the screen, & the people up on the screen are in the hands of the actor
They just strike me as uninspired actors, who had no investment in their characters, or they'd have found a way to make something
more of them