First time hearing about Matrix standard. But in case the clients are maintained by some individuals - why should we trust them that there are no backdoors in their compiled binaries? Seems like a nice project! But probably it will take it's small niche, probably for now it is not wide spreaded, haven't heard any noise about the standard.
The signal server has 10 maintainers out of which 5 are signal employees.
Matrix is 100% open source, has a larger community maintaining it and is federated.
If you're worried about backdoors then you should have more eyes on the code.
I was wondering why nobody seemed to mention Matrix here