Wesnoth is a pretty fun multi-player game. With a decent map design and thanks to them finally fixing the broken RNG it can be lots of fun to take the now reasonably balanced factions and fight with them.
That said I consider Wesnoth a fundamentally broken design when it comes to the single-player game, for exactly the reasons you state. It has many of the bad aspects of a rogue-like without many of the good aspects, and frankly good story design is next to impossible in it. I've tried writing a few scenarios, the WML interface started out awful, and is now much better, but the core problem with the game remains.
First randomness should never be a core element of game design. You can use it to add a bit of spice, but you should never win or lose a game because of luck. in multi-player this is less of an issue as a best of three between two players will almost always be won by the better player, but in a 12 scenario campaign one streak of bad luck at the wrong time results in either save scumming, or restarting the scenario.
If moving a unit to a location is the best possible move you can make in a scenario where things are about even between two players it shouldn't be possible for the consequences of that decision to utterly screw you because of unlucky RNG calls. In Wesnoth this can happen all the time because the need to maintain veterans all but insures you cant afford to lose things like your high level healers.
Crafting a story line in Wesnoth is next to impossible because it gives you very few options to insure that the player is ready for future challenges. One of your late game scenarios require they have lots of impact damage to survive? Well you'd better contrive some absurd reason for them to only be able to recruit that in this scenario so they can level a few of them up. You end up with absurd video game logic where you have to artificially constrain the player just to hint strongly enough at the needed force compositions.
The game also has extremely non-linear difficulty because being slightly worse at the game gets compounded. If a scenario is made slightly easier, then the campaign ends up much easier because the slightly better resources you get out of that scenario allow you to win the next with fewer losses, and so on. As a result many of the mainline campaigns are either absurdly easy or incredibly difficult.
Finally character development is very difficult. The game forces players to split their army in to bits they give a crap about and cannon fodder. The bit they are supposed to give a crap about is usually too large (~7 would be a good number as this is an amount most folk can keep track of at once, but in most scenarios it is closer to 15 or 20 and it has to be to face the diverse range of threats bigger campaigns throw at you). Meanwhile the low level units you use to screen just become faceless entries in a spread sheet because you bought them specifically to die in your better units place. As a result the player becomes completely detached from the story elements and character. I frequently forget what I'm doing in a scenario outside of the current objectives because world building is virtually impossible.
And the community doesn't respond well to these or any other criticisms. They like the random element, they don't seem to give a crap about characterisation, world build, lore or story telling. They are focused on the mechanics within a scenario. Well if that is their gig then that is their gig, but I honestly regret playing many of the campaigns I've played, I'd rather I had done something else.