Like the Aeon Obsidian, the Ilshavoh is a markedly heavier unit than its peers, reflected by both its cost and its power. Fast-moving, tough and destructive, the Ilshavoh is one of the strongest units available in T2 and is almost comparable to some T3 units. The Obsidian is the only T2 Unit which is stronger, costing the same as an Ilshavoh. It can quite possibly remain in good use even when the Seraphim player is into T3 (See the page on the Seraphim Othuum siege tank for more information) due to the lack of a light land raider comparable to the UEF Titan or Cybran Loyalist (the T3 Othuum siege tank is, like the Ilshavoh itself in T2, a heavier unit). Contrary to the opening cinematic of Forged Alliance, you should not expect Ilshavohs to be able to defeat Titans in direct combat. (According to calculations, even firing while running directly away will not allow Illshavohs to defeat Titans in 1-on-1 combat due to too much speed, health and firepower superiority for the Titan.)
However, mass for mass and energy for energy, it can rival heavy assault bots and armored assault bots, especially Percivals; for example, except for slower speed and having a greater range, three Ilshavohs can do exactly what two Titans can do, at only a slightly greater mass cost and a much cheaper energy cost.
They are also excellent for taking on tech 1 ground forces. Their extremely high rate of fire (reducing the opportunity for overkill or loss of potential damage due to missed shots unlike their Aeon counterpart) and good DPS allow them to cut through T1 tanks and bots like a hot knife through butter. An Ilshavoh will destroy a Cybran Mantis every 3 seconds, whereas it takes the equivalent of almost one and a half minutes of Mantis fire to down an Ilshavoh. Given that an Ilshavoh only costs about 6 times the Mass of a Mantis, this makes a rush to T2 ground an attracitve strategy for a Seraphim player.
Bottom line: Do not ignore these units, even at tech 3. A Seraphim player without Ilshavohs is a Seraphim player who is probably losing. The Ilshavoh is to Seraphim as Hydralisks are to Zerg in Starcraft 1 (and Brood War), the bread-and-butter unit of any army.
Note that unlike other factions, a "Experimental Firing Line" Strategy (a line of around 20 Direct-Fire experimentals advancing at once with AA support) will not work well for the Seraphim, due to the Othuy that spawns once a Ythotha has been destroyed, therefore, a tide of these units is a highly useful strategy even when you start building experimentals. However, keep in mind that Loyalists and Harbingers have a slight range advantage and significant damage advantage, so when going up against T3 or experimentals, Othuums are a better investment, leave the Illshavohs for tearing down buildings.
Strangely, despite being a Bot, Illshavohs have a Tank Icon when viewed in strategic zoom. This was most likely done to show that the Illshavoh is the Seraphim equivillent to a heavy tank. However, the Cybrans have a Tech 1 Assault Bot instead of a Medium/Light Tank, and it still has a Bot Icon.