Epiphone make some great guitars for the price, but the fact that they are so much cheaper means they have to use cheaper components to build the guitar. The wood will be of a lower grade and so will the electronics and also the build quality of the instrument when compared to a Gibson guitar of the same model.
But that's not to say some of the cheaper guitars can't be great. I personally have an Epiphone Wilshire that is stunning to play but i know the neck is a two piece instead of a one piece like on the original which is a common cost cutting method and the body is also a two piece. But to play, it feels great and i've upgraded the pickups so it also sounds great now.
I also have a Squier strat that has had a lot of upgrades to it, new saddles, wiring, pots, switch, jack socket, new capacitors, new pickups, full set up including a fret dress. I did all this as a bit of a project cos i wanted to know how good i could make the guitar and it plays and sounds really, really nice now. But when i compare it to my Tokai Stratocaster that was made in Japan in 1985 the Tokai blows it away, the build quality of the instrument is just so much better. I've changed the pickups in the Tokai as well because i didn't like them but that's all this guitar has had changed, everything else is original.
The main thing i think is important is the quality of the guitar build and the components used, you can change the electric components on a guitar at a reasonable cost so they are always the least of my worries.