I think the downside to Epic: Armageddon is that the Titan rules are really bland. They were such a central part of previous editions and EA reduced them just other unit: simply roll to hit and slowly tick off hit points until they are dead. Both Space Marine and Epic: 40,000 had lots of neat charts and random effects that were specific to certain Titans and it really made them come alive on the battlefield. EA Titans are basically just big tanks. Although EA is a great game, that fact just kills it for me.
Of course, the downside to Space Marine is that, 25 years later, it's just a weird game. Dreadnoughts have autocannons, Eldar Exarchs come in squads, Wave Serpents literally ride warp energy waves, Falcons carry two stands, Imperial Guard infantry do not ride in Chimeras and Space Marine tactical companies have six stands each of Devastators and Assault Marines. And there are all these weird units that never existed anywhere else—giant, flaming fondu trucks of Khorne, Ork bubble cannons and Imperial Guard jump pack troops. Space Marine feels old, too old in fact, as if it represents an edition of Warhammer 40k universe that never was.