I haven’t found any issues with it. The problem with making their own is that the MS Office file formats are purposefully poorly documented with the spec being over 6000 pages long and MS Office itself not adhering to it. It would be best to start with something where most of the work is done for them.

Is Onlyoffice enough for you?

Onlyoffice uses a different engine. It has much better MS office file support but worse ODF support.

They should have forked Onlyoffice, however I appreciate the effort to make their own from scratch.

Why don’t they fork Onlyoffice for this?

I hate them both. Librewolf and Ungoogled Chromium are the true private and non-profit browsers. Firefox and Brave are full of bloat like Firefox’s ads and tracking and Brave’s Wallet, News, Talk, VPN etc