I know the process is a little bit more complicated in this direction, and grub has to be updated and/or repaired after installing Windows. Is there another reason, besides the more complicated installation process, that it is not recommended to install Windows on existing Ubuntu? Does it lead to stability issues or anything once the installation is completed?
It is possible for sure to install Windows after Linux, and once everything is set up, it technically does not make any difference which OS was installed first. The only reason it is recommended to install Windows first is because the latter is a bit more easy.
- If you install Windows after Linux, your system will not display the Grub menu, but directly boot into Windows: you will have no access to the Ubuntu installation. It requires extra actions to restore displaying the Grub menu again. With legacy boot mode, users were required to reinstall Grub. With UEFI boot mode, which is standard nowadays, it is sufficient to change the bootorder in BIOS to the Ubuntu system. Still, this is an extra technical hurdle to users.
- If you install Linux after Windows, the Grub menu is automatically set up. The user has access to any operating system that is installed on the computer without extra interventions. Thus, it is easier for technically less proficient users.