This is a bit of a "how long is a piece of string" question, security is multifaceted.

From what I understand, it uses your phones kernel, so if its out of date or vulnerable, that might be a problem, and you may not be able to fix that.

Conversely, its running inside android, so the android hardening might make it more secure.

What are you specifically concerned about? Firewall? Zero days? Antimalware?

android is very sandboxed, so anything you do in termux/userland can't affect the android on your phone (unless you're rooted)

How secure an OS is depends entirely on the configuration. A Linux install can be less secure than Windows or macOS, if configured so.

Linux tends to be more secure OOTB because distro devs tend to be security conscious. Android is also fairly secure, since it has no root access, sandboxes applications to a degree, and has other hardening employed. However, Android is also very vast and built for various devices by many manufacturers, so it also depends on them.

There is no simple answer. Its is almost entirely dependent on implementation. All systems are vulnerable to things like supply chain attacks. We put a lot of trust in phone vendors, telcos and Google.

If you are going to compare to something like termux you need to compare with an equivalent sandboxed environment on regular linux, like a docker/podman container with appropriate permissions. As far as I know they use the same linux kernel features like cgroups and namespaces under the hood.

Traditionally Linux desktop apps run with the full permissions of the user and the X window system lets apps spy on each other which is less secure than Android sandboxing by design. There have been attempts to do better (eg flatpak/flatseal, wayland) but they are optional.