The UK government has guidance on secure usage of Ubuntu. It appears to be newly-written.
Lots of useful information, and it mentions that Secure Boot is only active at some time: nice to see that level of detail.
Secure Boot section:
Secure boot validates the bootloader, kernel and kernel modules. However, some boot-related files are not protected by default and could be modified by an attacker to tamper with the boot process. Hardening of the boot process can help mitigate the risk.
Ubuntu does not use any dedicated hardware to protect its disk encryption keys. If an attacker can get physical access to the device, they can perform an offline brute-force attack to recover the encryption password.
Encryption keys protecting sensitive data remain available to an attacker when the device is locked. This means that if the device is attacked while powered on and locked, keys and data on the device may be compromised without the attacker knowing the password.