I too have to join in the chorus of complaint about this scenario. I've seen several bugs; Urza moving right next to a stoned elf; undead units attacking one another; and, not a bug, but I've lost count of the number of times a second level elvish marksman has taken Urza from unhurt to 0HP in a single turn. Very, very frustrating. And even when you're using prior knowledge to avoid the ambushes, all it takes is a mis-click and a unit (particularly a mounted unit) to follow a different route to the intended hex and you'll trigger the ambush anyway.
Also, if the elves are looking for someone, why are they sitting on their butts waiting for the fugitive to come to them? (When that's not going to happen because after like fifty play-throughs he knows all the ambush hexes he has to avoid.) Shouldn't they be like, actively SEARCHING for him?
http://www.catb.org/~esr/wesnoth/campai ... howto.html gives guidelines on mistakes to avoid. This scenario badly breaks the advice about not using 'landmines'.
It even finds a brand-new mistake to make that I haven't seen before; either I control Urza or I don't. To have him running into the middle of a fight I'd just used other UNWOUNDED units to trigger, after I'd positioned him where I wanted him (you know, on a VILLAGE, because he needed to HEAL) was very very irritating.
However, I am cool with the undead attacking one another; that seems entirely fitting behaviour for uncontrolled undead. Though I did get confused when I couldn't tell from the tactical map which side they were on. (I gather this is a side effect of the WML machinations discussed elsewhere.)
Suggestions; foreshadow a single set-piece ambush; use mostly scouts and rangers, but have them actively looking for the PCs; and once the characters have been found, the elves say 'Ha, we've found you! Now our warriors are going to hunt you down like dogs' or somesuch and THEN form a force (or perhaps two or three for a pincer movement) to hunt down and wipe out the characters. If the characters spot the scout first, perhaps they need to stop him reporting back to base somehow. Also consider making the undead into random wandering monsters that attack anything they meet.
I also found the monster recruiting patterns odd. Sometimes I saw shadows and wraiths vey quickly; other times slower-moving skeletons and ghouls would show up several turns later. As I'm always recruiting the same units, I don't understand why the undead recruitment behaviour was so variable.
[Edit: Further thought: Perhaps Urza's AI-controlled movement from point to point should only trigger at the end of a turn. Having it trigger in mid-turn (e.g. when crossing row xx) means there's a constant risk it will go off at an inconvenient time. If he only ad-libbed at the start (or end) of a turn his behaviour would be a bit more manageable.]