They will be offering a premium subscription offer for more bells and whistles other than the free option...I don't know anything about user betrayals conducted by Beeper.
You have no way of verifying that the client is only doing what it claims. The Open Source community is highly suspicious of proprietary software, doubly so when it's based off of Open Source code.
If youre okay with that then no worries, but ofr myself and many others it's an absolute deal breaker.
To be fair, the client they provide to make bridging more accessible is proprietary, however you can fire up a fresh copy of element and connect it if you want and just use the text interface.
The clients are closed so that they have something to sell and profit. Not everyone can afford to give their time away for free.
But I (and probably most users) don't know them, over time people come and go, some even change who they are, businesses get sold. Only open source persists.