Drilling joists is very rare and absolutely not standard practice for any rigid pipework. It would involve an awful lot of extra joints in a retrofit situation so may be argued to be not altogether better. It is what I personally prefer as pipes are easier to insulate, but I have not been able to consistently use this method even in my own house. In fact, I was taught at college that it is not possible to drill joists and run 15mm plus diameter copper tube through them in retrofit situations (this isn't entirely true and, me being me, a long discussion followed). Depends very much on what other services are already in place and of course you cannot drill a hole in proximity to an existing notch, nor is it good to make new notches and holes if there are exiasting ones that can be re-used. It is the hardwood floor installer's job to avoid hitting the pipes and I would expect the person laying the floor to be able to work around perfectly normal pipework notches. Nailing through a pipe would be bad carpentry, not bad plumbing.
Of the soldering pictured, much of it is perfectly acceptable. It is entirely normal to have a slight drip under a horizontal joint and far better that than risk a dry joint. I agree others of those solder joints pictured are really messy. That said, I know an old plumber who, back in the days when solder was considered expensive, was pulled up on 'going through a lot of it'. He replied, 'I don't have have many leaks though, do I?'. His employer said no more on the subject. Lead solder is still allowed to be used on heating systems and has a reputation for being less brittle than lead-free solder. Indeed, I still carry both types in my van.
Failure to clean the pipe after soldering is less impressive and suggests a rushed job. I have seen pipes with excess flux left on the outside of them that have still been providing good service after 40 years, and if your system was properly flushed during commissioning to remove any excess flux inside the pipes I'm not quite sure why the pipes would pinhole, but I would still want to see the excess flux cleaned off,
My biggest concern with the installation quality in this case is pipes being run for long spans unsupported and poor design of pipe layout relying on the inherent flexibility of lengths of copper tube rather than proper bends being formed. Bend it around old pipes if you must, but use a bending spring or pipe bender - not just curve the length of pipe like a banana. Any plumbing textbook will apply the standard clipping distances of 4' for 15mm tube and 6' for 22mm tube on a horizontal run and I would not expect to see those long unsupported runs you have shown. Also, good practice for a pipe laid in a notch is either to fit a pipe clip every couple of joists of so, or to felt-line the notch to allow for movement. I agree that pipes should not be touching one another at any time.
Basically, if you'd bought the house like that, I'd suggest to snag the parts causing you noise or fouling the floorboards and leave well alone. For a new installation, this is poor, however.
