Percussion instruments by their very nature will obviously go out of tune because they are being struck with beaters. Additionally, wooden note bars in particular are subjected to atmospheric variations which also adversely affects the tuning. Finally there are all those instruments that have just never been tuned properly in the first instance.
When I first set up on my own the very first machine I bought, and the most expensive, was the best tuner available. Since then I have continually invested in equipment to enable me to do a better job, for instance I now have microphones, signal splitters, multiple tuners, tone generators, even strobe lights. Then there is the machinery dedicated to tuning, most of which I have had to make myself because they are just not commercially available. Finally, now I have built my own workshop, I even have a specific room to do the tuning in, designed into the plans so that my climatic control systems can be more effective as opposed to them struggling to modify the whole workshop.
My attitude is simple; I am an instrument maker, therefore the ability to tune instruments to the highest standards is at the core of what I do.