In theory, Asymmetic lines may make it easier to hit the uplink capped speed. But since most people would value downlink speed more than uplink, plenty of consumer packages have uplinks a fraction of the downlink speed. No major issue actually, if one actually monitors his total TX(up)/RX(down) data, most of the time they will find that TX is not more than 20-40% of total RX.
Ie. You only need to send a few kB of data in order to load a whopping 4k HD video on youtube, even throughout the course of streaming the entire video, data sent is just a fraction of data recieved. You do not send 400MB of data to receive a 400MB video. Streamyx was AsymmeticDSL (ADSL). Even on copper, as long as everything from the user to ISP functions properly, it is what it is. Again this only speaks for 'the common end-user'.
From my early understanding, the disadvantage from an end-user point of view is when the uplink is saturated. This can lead to bufferbloat.
Ie. If a user hammers the connection with torrent uploads, latency may rise, sometimes up to 1000ms depending on the device used. Yes, the downlink is subjected to bufferbloat similarly and the effect is bad for VOIP/Sykpe/Gaming or other real-time applications. Yet, it is somewhat a little more undesirable on the uplink. As how TCP/IP works naturally as a two way street, if you cannot send anything out, you cannot get anything in return. If you delay sending something out, you delay it's return result too.
Can be somewhat mitigated with 'traditional' QOS. Can almost be completely eliminated with CoDel type 'SQM' QOS. Then again for most people, the effect of a significantly lower uplink speed is negligible. They are more likely to hammer the line with incoming traffic and cause the same situation, but on the downlink instead. So in conclusion, sym/asym is merely a difference of a coupled up/down speed or independent up/down speed. There may be some technical differences but so far, have not heard of it affecting user experience in a significant way, not on fiber at least, unless someone screws up badly somewhere beyond user control.
Sifus, do correct me if i'm wrong of if there's additional info.

Noted for read.