The Salem Class Destroyer is a Cybran naval unit. It is armed with a direct-fire weapon, an anti-air cannon, and torpedo tubes. The destroyer can gain the ability to move on land via the Land Emergence Galleon Systems upgrade.
It's a very formidable unit with various features. With a movement speed upgrade, it's fast enough to dodge some direct-fire attacks, and becomes good at chasing retreating enemy naval units. It also has torpedo tubes to catch subs and ACUs hiding in water. Its cannon outranges Point Defenses and packs a punch. The benefits of long-ranged weapons are considerable; the Salem enjoys bombarding coastal bases and short-ranged land units without retaliation, or retreating from incoming enemy forces then shelling them again from a distance. The Cybran LEGS technology allows it to retreat to the ground to escape a submarine attack, and retreat again from land to water to avoid massing land units, which in turn, can effectively raise veterancy levels quickly. Unfortunately, its main gun can fire upon only targets in front of it, meaning that it cannot attack units chasing it or outflanking it. This is a major drawback for the Destroyer. It won't be able to kill units pursuing it before they catch up, so unless the chasers are crushed by other units or diverted to an alternate objective, the Salems will be caught. Beware the Illuminate, as most of their land units hover, and taking to the water is no defence.
Sadly, the Salem has its weaknesses. It is by no means tough, and while it is a great defensive unit early on, two or three of them do not make an effective army. Large numbers are required if you want them to do anything, and at that point, the Executioner Battleship and its obscene range seem a better choice (Executioners cost a lot more, but only take a few seconds longer to build). Preferably, Salems should keep their distance from enemies at all times, if you want them to survive. It doesn't take many enemy units to overwhelm a Salem-class, despite their size when walking on land. Use them to supplement existing forces or defensive lines, not as a mainstay.
Whilst powerful and with no obvious weakness, Salems cost the same mass as an entire land factory and more energy to boot, do not expect to posses a huge amount of these as you would your land armies.
The destroyer makes an excellent main combat unit. Its powerful combination of range, power, and versatility make it the backbone of most Cybran fleets.
Though a powerful unit, the destroyer lacks health and armor. Using very large numbers of destroyers without other ships nearby is more expense than it's worth. Instead, add battleships for offensive or combat groups, aircraft carriers for support, and engineers for repair or expansion.
With the Land Emergence Galleon System (LEGS) upgrade the destroyer gains the ability to move on land alongside Cybran land units and the other walking ships. Commanders may be tempted to bring the seemingly large and powerful destroyer as a main assault unit, and in many cases this is a useful tactic: destroyers can effectively shell a base from outside the range of point defenses. However, in some cases this is a very poor choice: moving a Cybran fleet onto land exposes it to fire from enemy land units, particularly tanks and assault bots. Instead, have destroyers and ideally battleships bombard and destroy land forces and structures as much as possible from the water, where enemy tanks cannot pursue, before clamboring onto land to complete the battle once victory is guaranteed.
That said, Salem destroyers make effective defensive units in the early game. Shielded by Adaptors, they can be reasonably survivable, and have a good range as well as an anti-air attack, complementing your point defences nicely. A couple of them supplementing an existing fortification line or guarding an outlying base under construction are useful. The Executioner Battleship is better due to essentially being mobile artillery, although the Executioner's range means it is rarely, if ever, disadvantaged by remaining in the water.
While it does open them up to being assaulted by enemy land units, moving onto land is a good way for Cybran ships to escape subs. It also allows them to shelter from heavy enemy retaliation (such as gunships) amid an army or point defences.
Salems are great for taking out early UEF Mastadon rushes, due to the main cannon and torpedo launchers both being able to target surfaced ships at the same time.
With micromanagement, a Salem can avoid direct-fire weapons, as can many reasonably quick land units, especially at range. It won't have much luck dodging Rock Head gauss cannon blasts, for example, but Fistoosh missiles can be kited and avoided. Direct-fire cannons aren't guided; if the target is moving, the firing unit calculates the target's expected location and aims at that calculated location. Should the target change its course when the fired shell is in the air, then it may well miss. You can observe this when combating Battleships, as their shells stay in the air for a long time. You may even be able to defeat a force of Battleships with similar numbers of Destroyers, but it does require concentration.
The Salem Class is a very powerful naval unit, but should not be expected to win wars in small numbers without support.
The Salem Class is a better anti-navy unit than an unupgraded Mastadon. The Mastadon is a better anti-air, ranged and shelling unit when unupgraded, and almost matches the Salem in naval power when upgraded.
The Salem Class has the lowest range of any non-submarine naval unit besides the Command Class.