It can easily happen that a LLD rule has an error state, the red "i" icon and a pop-up message is there.
But it's not clear how that is actually critical.

In some cases it's not really possible to get rid of that. A real use case mentioned in ZBX-26664 (see very last comment).
That's what I call as a non-Blocking error state, as the rule is actually performs its job for all possible objects/properties.

But another story if a rule has errors (different cases) like:

Invalid discovery rule value: cannot parse as a valid JSON object: invalid object format, expected closing character '}' at: ']'

Cannot process LLD macro "{#IP}": JSONPath query must start with the root object/element $..

and this is what I'm calling as Blocking error, as the rule fully not functioning.

From regular user perspective (I count myself here as well) - its basically not clear at all what to expect from the rule? Is it still doing something or it's already failed completely. This is specially hard to understand if you already have some objects discovered by it in the past, so you could not test/force it to understand influence of the error.

What do I see could be improved:
The error message could have a 1st dedicated line, cleanly indicated what is "severity" of the error and how it's critical for zabbix user.

For example for the Blocking and non-Blocking cases, the title could be:

Critical failure, rule is not functioning.

Rule is functioning, but some objects have failures and may be not completed/updated.