Yeah, yeah, I know about the Typhon and so on and honestly, I just forget about the Diomedes, or the other new vets.
This is a rather personal thing, since I'm in love with the Deimos since I first played the game after it was released^^
Well, it's good that you love the Deimos. If you've ever seen the Sol Expeditionary Force picture I posted, you know that the Tevs can field what amounts to a
wall of Deimos corvettes. The Buntu actually have probably twice as many again Sanctus cruisers (at the start of the WiH 1 campagin) but honestly 1 Deimos ~ 3 or 4 Sanctus cruisers, in terms of offensive capability.
Deimos corvettes function, if you want to think about it in terms of contemporary armies, as an IFV rather than an MBT. They're cheap(ish), flexible, can deal with smaller opponents, provide cover and fire support for infantry (fighters), guard more important units, carry out offensive operations against not so well defended targets, and the like.
Diomedes function more as Karunas - light carriers with frightening firepower, able to carry out small offensive campaigns all by themselves. A Diomedes stationed in the Kuiper belt could, for instance, launch squadron level attacks sporadically throughout the system without being detected, taking out, for instance, an upity Elder, while also being able to maul a Karuna in a close range battle. This is pure conjecture on my part, but given what we know of Tev vs Buntu technology, the Diomedes is most likely able to go much longer than the Karuna without resupply as well.
Chimeras basically exist to spearhead major offensives (with triple mounted heavy beams) and to defend high value targets (with pulse guns and
7 AAA guns, but are apparently not cheap, nor are they extremely flexible. They're a lot like MBTs really.