Yes, touching the internal flash when you don't know exactly what you do is dangerous. Using nandwrite -j by mistake is recoverable but not easy (special hacked kernel, special flash_eraseall) http://www.gossamer-threads.com/list...rs/10530#10530
Reflashing config or bootloader partition may be unrecoverable. I should probably comment out -j option in nandwrite source and rebuild the binary sometimes since it is quite useless but very dangerous on nokia tablets.

As for normal writing to the internal flash I wouldn't be so afraid. It would take some time to wear out internal flash. See end of https://maemo.org/bugzilla/show_bug.cgi?id=598