Are these not suitable for building an external SaaS web-app? What exactly is the limitation that makes them not suitable for a world accessible SaaS app?
Mostly that they can't figure out how to price it. The tools typically charge a lot per user ($20+/month/user). If you make external users cheap, then customers will just enter internal users as external users.
on the builder side, you can expose more confidential/internal data since you can safely assume the person and intended audience are only employees.
you can move faster if you assume away requirements that dont really matter in contexg.
You become reseller of the "low code tool" and now what are you providing to your customers that they could not do themselves?
There is also problem that anything goes wrong and you are not able to properly support your customers. With SaaS you have the code you have the team, you built the tool that you support and features are providing value.
I don't think you can do "multi tenancy" on "low code" platforms you would have to make separate account per customer. Which means you only repeat the work as you probably won't be able to build once and provide stuff for multiple companies.
GDPR where you are not side in processing might be nice as you ditch responisbility - but then your customers need to know who has their data. If they know what is stopping them from simply using "low code" on their own.