Legolas was doing a fine job climbing a moving mumak and dodging fire and shooting back with accuracy... of course he is PJ's legolas, can't be touched.
I would be fine with the +1 option in some friendly games but not siege games. Only because most of those the defender has front wall rank of just bows, there is already a slim chance of climbing without the ladder being knocked down, even slimmer chance of making to top and winning the fight to climb up... so to have to also have even less chance of survival due to bows would make me not want to even bother with ladders in the first place.
In friendly points match games and the like, I would be ok if the forces are close in shoot value. IF they are not and only one side is likely to be climbing due to terrain, then the bonus could have a greater affect. For example Elves with elven cloaks on high ground would be untouchable. Practical auto hit on any climbers to get to them and you could not shoot at them from a distance. they would be camping snipers for the whole match.
These things are just 'considerations'. The bonus in most cases would be OK, at the same time, ten years the climbing rules have worked as is cause it is not used often.
For prone models or knocked to the ground models, the target is smaller right? they are laying down and thus should be harder to hit... since they are already under penalty enough laying down, I don't think they need more penalties, though I would welcome reading options or house rules anyone had a good idea that is working for them.