In both cases, the builder really can do nothing to prevent it. These are risks that can occur.
Yes – again, I agree with your good posts. Sometimes you are powerless – it just is that way. Especially regionally different. In terms of all costs. Fell-Bert, for example, is extremely expensive when it comes to connections. I thought my pipes must be made of gold, with platinum and diamond inlays. Far from it. Completely normal...... I lost a lot of money, almost 10k, planned were 5.5k. Of course my stupidity (so much for intelligence, I could have asked, but I thought (but here it gets difficult), it could (could is always bad) fit.....so, if you think it might: Stop: Ask people who know, then keep calculating).
The biggest problems for me were the municipality, authorities (building delays without end because of that), and the ground conditions. Soil surveys are great to read, but they only drill at 4 spots......lol. Slate seams and rocks can lie anywhere. And they lie everywhere.
Neighbors next door had that across the entire plot. Others, 80 m further, only had mud; they had other problems.
ALWAYS: Plan 10-15k over the buffer as a buffer-buffer, absolutely. Something *always* goes wrong (only solvable with money). I had a good buffer and still needed about 30k more. The money just rushes through your fingers when you build.