A (relatively young) surgeon I once talked to told me there are 2 problems dealing with Scar Tissue.
1 - It turns to something akin to concrete which of course is hard to cut and
2 - is uniform throughout, in contrast to 'fresh meat' where the surgeon can tell where he is by knowing the different characteristics of different tissues. (my paraphrased version of what I heard).
This uniformity makes it more 'tricky' to avoid cutting veins / capilaries as they proceed (i.e. creating 'bleeders').
Anyone who can explain this better, please come forward and do so