The upgrade process from Windows 8.1 to Windows 10 is quite simple and seamless. There are a large number of users evaluating the Windows 10 Technical Preview who have performed the upgrade and have not encountered issues.
Some potential errors with the upgrade process can include drivers that are not compatible, software that interferes with the boot process or which emulates hardware, and the pre-upgrade environment not being fully up to date. If you wait until the official release of Windows 10, you will want to ensure that the drivers installed for all devices are listed as Windows 10 compatible by their manufacturers, and that the system is fully up-to-date.