Why do people say app-bandwidth and app-qos dont play well together?

I'm on ADSL with an relatively slow link. 4500Mbit down/1000Mbit up.

When the upload is satturated, the download slows to a crawl - this is well documented behaviour common to ADLS that has something to do with the router unable to process ack packets fast enough.

QOS is supposed to help - the default clearOS QOS gives highest priority to non-TCP packets (includes ack packets ) so it keeps up.

In practice I'm finding that QOS is not helping.

What does help is to install the old app-bandwidth and set a ceiling rate on uploads that is a little below the maximum (e.g. I throttle uploads to 900Mbit when the link is capable of 1000Mbit up)

This instantly restores the download speed.

So my Q's.

Why doesnt QOS help?
Why do people say app-bandwidth and app-qos dont play well together? I have them both installed now and can't see any miss-behaviour but not sure what I should be looking for. I'd like to keep QOS around as we run VoIP phones... but only if its useful. app-bandwidth actually seems to solve out VoIP stutter (due to people uploading) better than QOS did. What am I missing?