I think compatibility issues in LaTeX often come from packages that redefine the same macros in incompatible ways. This kind of things doesn't happen in typst because all user code is pure: a package can define 1) values or pure functions that can be imported (this makes them available only in the scope where they're imported) and 2) content that can be included in the document.
There's still potential for conflicts, for example content can contain elements that represent a state update such as incrementing a counter. Packages can define their own states for internal use, and the namespace is global, so they can interfere with each other if they don't follow good practice (prefixing state names with __package-name for example). And show rules can replace an element of content with another one, for example one package can replace verbatim code with a figure, and another package can format verbatim code. What happens if you mix them without limiting their scope?
But so far, I think the compatibility problems in typst are more of the "well, what do you expect?" kind. Compare to LaTeX where sensible code is broken when a package makes a small changes somewhere deep in the macro pile of cards.
For example someone here mentioned the example of one package changing "the" to red and another changing "the" to blue. This can be done declaratively in typst, and won't cause an error, but the result will depend on the order of declarations.