I have had several windows 10 installs that run with legacy boot mode and have never had an issue with them. You can boot it in Legacy mode, no problem.

Can Windows 10 be installed on legacy BIOS?

However, USB drive only boots in UEFI mode, and not in legacy mode (I alternated between the two options in the BIOS menu). When the Windows installation process is started in UEFI mode, it cannot install Windows to a MBR drive. Hence I need to create a bootable USB drive that can start in legacy mode.

Is it okay to use legacy boot?

It won’t cause any damage. Legacy mode (a.k.a. BIOS mode, CSM boot) matters only when the operating system boots. Once it boots, it doesn’t matter anymore. If everything works as expected and you’re happy with it, legacy mode is fine.