There will be almost no UX impact if the decision on what defines each microfrontend is UX-driven. For example, feed and stories in Instagram-like webapp can be microfrontends - UX defines the structure of the container app and UI integrations, but then they can evolve independently.
There will almost certainly be a UX impact in the form of performance though - the whole point of micro-frontends as I can see it is that it allows you to have different infra&dependencies in different parts of the UI. The only reason you'd want to do this is because you don't want to share the same infra&dependencies across the UI which inevitably leads to duplication of frontend infra.
e.g. if my whole app is React based, why do I need micro-frontends?, using them seems needlessly complex. It only seems relevant if for example I have a product where team A has a legacy jquery UI, team B wants to add a part of the UI built in Vue, and team C wants to build out some features using React - but in that case the user is now having to download 3x the JS infra code that they did before because none of the teams can agree on a shared stack.
Also this pattern would seem to make composability of the UI much more rigid and inflexible. In the IG example you mentioned, lets say stories and feed got built out using separate stacks with separate codebases and infra. Now lets say we want to add a saved stories unit to the profile page, or add some recent stories as a new unit in feed (real examples - I used to work at IG :)) If we were on a common stack like React, I could just re-use the React story reel component from the stories tray and drop it into the Profile page (& maybe tweak a few props). With a micro-frontend, I'd have to create a content area for the other team to put thier story unit, agree on what the expected interactions between the profile & story unit were going to be, agree on a contract etc. wait for the other team to adapt the story reel so it was usable in the new context, make sure they deploy the new version of thier story unit...
The only situation I can see a micro-frontend being beneficial is as a stopgap pattern while migrating legacy apps - it certainly doesn't seem like an ideal end-state to me.
I agree with you that the use case with different stacks is suboptimal and may make sense only in enterprise apps. For consumer apps the impact is indeed too high.
However, just like with microservices the main point is not about having different technology, but about having different lifecycle and deferring the component integration to deployment or even runtime.
