Because level 2 (and lvl3, lvl4) units are so much better than base units (especially since they only cost 20 gold to pull out...), you often times end up in a situation where the developers had either more units than you, or fewer units than you, at particular points of a campaign.
For long campaigns with like 20 levels, it leads to innate imbalances. If you managed to keep your units alive and coming back in all levels, you'll have a much easier job at the end than another player who lost a unit once or twice.
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The fact of the matter is: an Orc Warrior with 50% night attack bonus with 10 damage per swing x 3 can one-shot your tier1, and even tier2 units.
Your typical land routes have 40% evade, meaning there's a 21% chance that the Orc Warrior hits with all attacks and deals 45 damage, more than enough to kill any tier 1, and most tier2 units. And you have nothing you can do about it.
Even if you mitigated this issue to the max: say with an Elvish Sorceress in a Treeline (70% evade), there's still a 2.7% chance she dies. There's no higher evade than 70% in the game, and Sorceresses / Archers (with 70% evade) have rather low HP to boot.
So you're ultimately relying upon luck to survive the night-attacks by Orcs.
Losing that Elvish Captain or Sorceress early in the campaign very well could lead into cascading failures, an inability to complete say the 9th chapter (when things get difficult).
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Sure, there's a day/night cycle. But campaigns inevitably lead to night-only scenarios (ex: caves) where you have to just deal with Orc Warriors and their 20% chance to literally one-shot any unit of yours, with no way to reasonably beat back the night-bonus. (No forests exist inside of caves, so you have no opportunity to reach 70% evasion).
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Orc Warriors are among the more "fair" units as well. I recall maps with "lancers", who are daytime units and level 2, but with 12x3 attack. Except... lancers deal double-damage to all units (and all units deal double damage to lancers).
So really, lancers are 24x3 on the charge, before the daytime "sun" bonus kicks that up to crazy numbers. With a crazy 10 movement, you can't even outrun these guys or hope to fight them at the night time. They have the highest movement in the game.
As such, your only plan is to send units in, and hope that they miss. If lancers get lucky, your units die and I guess you just restart the level. There's no strategy to it: your units have 6 movement, and lancers have 10. You just gotta run in there and hope for the best.
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Strangely enough: I think luck in the PvP world is a bit more fair. I'm well accustomed to games of chance (Pokemon, Backgammon, Poker) in the competitive world. Furthermore: the PvP world in Wesnoth is mainly played with tier1 units (so no "luck factor" in determining if your tier2 units survive: because you simply don't have any tier2 units in battle). The games end up more fair. Maybe the enemy gets lucky and kills my tier1 elf fighter due to luck. Oh well, it was only 14 gold, and I can build another.
The campaign is where stupid lancers feel like you're just playing vs a slot machine however. A line of enemy lancers just isn't fun to go up against. There's no strategy involved in fighting vs it (and it also would never come up in a PvP setting, since Knights are often times a superior unit over lancers and are the preferred promotion path)