Judging actor performances is of course fairly subjective but in my eyes Beltran was the worst actor on the show. He was wooden and seemed very unmotivated to me. Like Montgomery in ENT (who was kind of the opposite with his annoying uber-enthusiasm) he got his ordinary amount of character-focused episodes but later he got less and less spotlight. He played himself "out" of the show while actors like Picardo played themselves in the foreground.
You rarely know beforehand whether an actor will shine in his role. Think about Bujold, without doubt a brilliant actress. But it was the wrong role for her and they realized it before they started the show. Sometimes you only realize it while the show is running and then throwing an actor out is a fairly extreme step while giving his character less spotlight is more moderate.
I agree that you need interesting characters but VOY was a John Doe show. The idea was not to show a new Klingon, a Spcok 2.0 and a Chekov-like greenhorn but to show ordinary people on an ordinary ship thrown into extraordinary circumstances. Not everybody on the ship can be an eccentric EMH or Paris.
And an interesting character can still fail. If you don't watch ENT and just read the character sketches Mayweather, the guy who is born in space, who spent most of his life on a cargo ship and unlike all the Starfleet folks knows some of what will be out there, is the most interesting one.
You can get a variety of interesting characters on a Trek show but then you have to go down the "static setting with serialization" path of DS9. People on a cargo ship or a colony or whatever are bound to be more colourful than disciplined Starfleet officers.