I think NextCloud app will be usable for most normal daily cloud transfers when the transfer volume is relatively low, but I needed a faster way to transfer large directories on my phone that have accumulated over the years! So, I moved large directories to my hard drive via direct cable connect to my Linux desktop HD (client), then dragged them to the NextCloud browser window (server). Transfer was much faster 300-400MB/sec, as opposed to 800K/sec.
Also the Android client version 3.7.2, as of today just isn’t reliable, even when I wasn’t low on space, it has a mind of it’s own. Sometimes transfers appear to stall. Sometimes it transfers what you want but often it doesn’t. You choose to transfer 50 files to the cloud, it decides what it want’s to transfer AND doesn’t warn you of that. It may move 5 or 20 or whatever but doesn’t warn you that it hasn’t moved everything, and that’s the biggest problem when people are depending on a cloud backup solution. What if I used the app to copy 50 pictures to my server, then assuming it had completed my request, deleted those pictures off my camera? Not good!