Limitations

General

No trimming and no NativeAOT. Templates are compiled and loaded while the application runs, which needs the C# compiler and the JIT compiler.
No sandbox. The code of a template runs with the permissions of your application. Never compile a template written by a user of your application.
Templates do not compose. A template cannot include another template, and there are no layouts or partial templates: include adds a C# file. Run the templates separately and pass the text of one as an argument of another.
Restricted types. The types of arguments, OutputType and the base class must be public, not nested and not generic. See Passing arguments.
dynamic code is never unloaded. See Avoid dynamic code.
A built template is frozen. It cannot be loaded again or reconfigured: create a new Template.
Indentation is kept. The spaces in front of a statement block alone on its line are written. See Line breaks.
Formatting depends on the writer. Values are formatted with the culture of the writer, which is the current culture by default. See Write a value.

Only the title section is returned; the content of other sections is not available to the caller.
Non-ASCII characters are written as numeric character references by #html and #attr.
#url encodes a part of a URL, not a complete URL.
The library generates the HTML body only: no plain-text version, and it does not send emails or attach the images.

Arguments passed with --arg are strings, and the arguments of a JSON file have the types of their JSON values: values of your own types cannot be passed. See Pass arguments.
Every template of a run receives the same arguments.
T4 templates are read with the syntax of the library: only the delimiters and the extension of the output directive come from T4.