Some (all? I have only tried a couple) of the !bangs exclusive to the ultimate plan have a different behaviour on the starter plan.
For example, I wanted to try out the !expert bang after beeing impressed by the !fast that is available for everybody. I was redirected to expert.nl with a search on their page. The same happended when I tried to use the !code bang: "!code Write a Hello World script in Python" redirects to searchco.de which times out (giving the impression that Kagi hangs, but that is out of scope here).
The issue is that the bangs mentioned above, !expert and !code, which are bound to AI functionality when on the ultimate plan is used as normal web bangs on the other plans giving an inconsistent default. As a user that has read the documentation about AI bangs and is excited to try it out I would not expect to be redirected somewhere unrelated when trying it out. I now understand that the !expert bang is only available for the ultimate plan but it took an embarrassingly long time to understand (tried it several times over a couple of weeks). It says clearly at the top of the Assistant doc page that the beta is only available to ultimate plan members but since the !fast bang works for everybody I was expecting the others to work as well (or give me an error about membership level).
If the user is not on the ultimate plan, show an informative message that the feature is only available to subscribers to that plan and don't redirect to the site search. This would also be a good oportunity to promote the benefits of the plan since the user a) already has some interest in the feature and b) might not know what other !bangs or features are available to subscribers of the ultimate plan. I understand that you might not want to break these bangs for the users that use them but this is still the issue with ultimate plan members and the web redirect pages seem to be down (!code at least) which renders it useless anyway. Maybe show a message for the user to explain that they can add a custom bang that goes to the expected site if they expected to go to expert.nl
The ability to use !expert and the others drove me to upgrade to the ultimate plan at least but I had no idea what actual bangs were available to which users from the documentation.
