Bear in mind, epic is a game of abstraction, most of the dreadnought builds are too granular for epic and have been integrated into the two main variants (there are others in variant lists too)
Dreads aren't great, although they work well when podded in with devastators, they are Armoured vehicles which are moderately less vulnerable than infantry, and also as they are vehicles, infantry can hug them for cover
Also in a game of titans, macro weapons and war engines, a comparatively tiny robot man isn't really a big player....
Preds have their uses, they are fast and don't suppress that easily, a formation of annihilators is nasty against most vehicle formations and even stuff like lone shadowswords should be wary, as the formation puts out 12 shots
Destructors are also quite useful against infantry, another alternative is to mix the types so you have some decent AT firepower as well as solid AP
This being said however, Marines aren't renowned for their shooting, so they don't generally compare well to other races in that regard..... they're more based around getting up close and combined engagements and air assaults
Also as said, an assault cannon dread has the same firepower as a combat squad (better AT) as well as a macro attack in CC, so broadly in line with my hazy recollection of 40k, they're also immune to all AP attacks
I still think they should be better, chaos dreads are fearless AND have better firepower, but that's a discussion for another thread
