I always stress that Mordheim is a CAMPAIGN system. As such its scenarios are not meant to be completely fair but to balance overall. Surprise Attack favors the lower rated warband, since that player gets to determine Attacker or Defender. It is meant to give that player an advantage. In the early stages of a campaign the advantage may not be needed or even fair but that's also why the system for choosing the scenarios is random.
I've played it dozens of times over the last 10 yrs and my experience is rather different, for us I'd say its more 60/40 to 70/30 (although some players like Da Bank always lost as the Defender.)
Terrain is huge. If there is a tall building near the center of the table 2 warband members might be able to set up inside of the same building. Either way our group tends to look for hard to approach places to place their heroes and then henchmen are shunted to the outside edges. Survival is the key for most of us so we also shove expendable models into the open as bait or even as sacrificial lambs so that the heroes can scamper if they want.
Pay attention to the rules. Most Defenders do not take advantage of the rule that allows them to charge as their entry move for the warband members who are off table searching the ruins. Granted the opportunity to charge is rare, but it is usually devastating to the attackers.
Finally I have to ask how long your campaigns last and how you treat new players and players that are starting over. We play at a store and run 14 week cycles. For new players who join in progress or if someone starts over, its sink or swim. Thus there are some huge disparities in warband strength. In those situations the lower rated warband almost always loses and its an uphill battle for them no matter what the advantages.