Huh? quick example - a customer-facing platform with a provisioning dashboard, and a user dashboard; they can (and should, for several reasons) be developed as separate applications, and will depend on different APIs. Are you saying having 2 distinct technologies on 2 distinct components of a product is a mess? Without any other details on the product?
A good example on the type of products with these separations are e-commerce systems; payment gateways; cloud-native SaaS solutions, etc etc etc.
I'm sorry to tell you this, but your comment just shows how deep your lack of experience is; any reasonable complex product using frontend technology will have different interfaces with different requirements, different levels of polishing and - frequently - maintained by completely different teams.
If one person writes code only in react and another only in vue, in the same product, you have a mess.
If one person writes their react code in vim and another writes it in an IDE, you don't have a mess.