We have just spend 2yrs renovating what had been a derelict, 1930's property. Overall, there haven't been any major disasters, but still costly and some very time consuming finds! We did expect things, but you can only plan for so many eventualities.
The previous owner used the garden as a tip. We dug out over 50 tyres, along with axles, windscreens, bags of rubbish and large sheets of asbestos! This was all hidden with 20yrs of overgrowth on top!
When digging up the floors- they found a room which had been an old extension- was not actually connected to the rest of the house and apparently 'floating'.
The sewer pipes went to the back of the garden, did a 180' turn, then came back to the front again. It hadn't been done correctly when they got rid of the cesspit- so they had to all be replaced
Lots of DIY, quick fix jobs found. Newspapers and clothing stuffed into door frames and holes, dangerous DIY electrical connections!
Our workmen were generally great, but as you said- you really don't know if you are being overcharged, or they are stretching a job out longer than really required. 1 lot over estimated, and we've been left with 6 tonnes of sand and 2 tonne of bagged concrete!!!
Some things I personally couldn't stand- workmen radios. Some days, we'd have 4 of them dotted around the house all blasting out till I told them no more! Despite skips and bins- I'd find crisp packets, cans, cigarette packets etc stuffed into pipes, bricks and any place the CF's could be lazy and try hiding them.