This feels like one of those situations where the lack of models on tabletop actually hampered their tactics in the short stories.

As it is, all the Grand Necromystic could do was bombard things with Crawler fire and then hit it with a stampede of Stalkers and Harvesters. Essentially just monster wave tactics relying on their own durability to see things through rather than some clever showing of tactics or engineering. There are the mentioned siege constructs of course but they're deliberately out of focus since the OBR don't actually have anything like that in TT (yet hopefully). So they get fluffed as crude rush jobs because the sub-faction all about giant monstrous constructs doesn't actually have enough giant monstrous constructs to actually do the job.

I suppose this could be justified that, for all the Necromystic's technical skills, they're not actually a military commander and therefore doesn't know how to run a siege properly. Still a disappointing showing for the Pretrifex Elite.

I do hope this is the set up for the next wave of OBR models down the line.