One thing to note with migrations (and simple upgrades of the OS) causing issues, particularly from Tiger, is that there are some apps folks are still using that do put their junk in the system folders and many more that just are not going to even run in SL without the app being upgraded and some never were.
I have to believe that the majority of migration issues are caused by such items.
Another reason I encourage a bootable backup. My own upgrade to SL from Leopard caused a kernel panic on boot up. After attempting to start the machine twice, booted from my backup, read the error log, found the 3rd party kext that was causing the issue and removed it. Took me about an extra 10 minutes to fix something that otherwise might not have been possible without wiping the machine and starting over.
To 'expect' there will be zero issues when upgrading your OS (or importing all your apps from a previous OS) whether that be OS X, Windows or Linux is really unrealistic. I'd guess that better than 90% of all OS upgrades/migrations go perfectly fine. But to think that the OS developers have the time or the inclination to test every single piece of 3rd party software out there and then notify those developers and to then wait for them to fix their apps prior to an OS release, well...