I kind of wrote a lot but didn't summarize very well. A key principle of UDP is quite the contrary of what you are saying. UDP applications can have guarantees

I kind of wrote a lot but didn't summarize very well. A key principle of UDP is quite the contrary of what you are saying. UDP applications can have guarantees that they don't have to engage in MTU discovery or fragmentation issues. It provides a way to have an abstract, static contract about packets that is agnostic to layer-2. Because UDP is comparatively simple, it has been abused as a proxy to implement your own protocol on top of IP, and in that context you of course you really have to deal with al those concerns. However, it is a terrible mistake to think that is what UDP is about or how one should use UDP.