"Can I install Ubuntu touch on any tablet?"
No
"I don't see any reason why it wouldn't work with any device."
There are a myriad of reasons, like different screens, chips, RAM etc, the same reasons you must have Android built for that specific device, not just any old Android rom.
"Is it risky? Does it work? If not, why?"
Yes it is, there is the same risks as flashing anything, bootloop, brick etc... See answer above for the why.
"Why is there no instruction on Ubuntu's website about how to install
touch on a tablet?"
There are
"And more generally, is it or not possible to install Ubuntu touch on
any device? If not, will it ever be?"
It will never be possible to just install on any device, not all devices are created equally and compatibility is a big issue. More devices will get support in the future but never everything.
Although, if you have exceptional programming skills, you could in theory port it to any device but it would be a lot of work.
Info on porting touch to your device and currently supported devices, plus 3rd party roms on XDA.