At my level, The Valiant is perhaps my most used T2 vessel. It has good HP and DPS, Which makes it very effective for dealing with Frigate spam. The UEF navy is often trying to make an early tech transition to get their Battlecruisers out. Which often means an early Valliant + Frigate combo is necessary to hold parity with the Frigate spam until I can field Battlecruisers.
When dealing with other factions destroyers, the answer is nearly always to go for a Frigate heavy force and charge straight in. The Valliant has very favourable turret position, firing cycle, and projectile speed for micro. As they have a good chance of hitting their targets, and no blind spot. The answer to fighting Salem is to let the Salem kite, but in doing so move behind the Salem and enter their blind spot. Salem has only a forward mounted turret and can't shoot directly backwards.
The Valliant is not meant to be used like other Destroyers. Think of it as a T2 Frigate.