@Jiefu. Are you refereing to the F@ST3864 (2.4 GHz WiFi, only) or F@ST3864AC or F@ST3864AC V3 - or all three?
I sympathise with your post and believe that Optus have constrained the user interface so that the LCD can use it. I suspect that there a lot more satisfied users than the remainder (on that basis). Optus are remiss in not providing VOIP details: they get only bad publicity as a result. There must be a reason and they would be wise to state it. However, if you don't want their VOIP, you can use any modem-router.
@leeroy88. People tend to post negative reviews.
Furthermore, and in defence of the 3864 (NB: AC variant), I changed to NBN FTTN in Oct 16 in a Central Tablelands NSW city. I elected for the RR plan at the time (100/40 + Fetch Mighty) and received a F@ST3864AC. My location is Telstra-friendly: consequently, Optus (4G) mobile and fixed line services are excellent (and heaps better). I am in an extremely quiet WiFi location and am <200 m from my node. In Dec 18, I changed plans (now 50/20) and received the F@ST3864AC V3.
The 3864AC was flawless for two years. I connected everything to it by wireless - a LAN printer using 2.4 GHz and everything else using 5 GHz. Recently, my new neighbours connected to it using my 2.4 GHz while waiting for their NBN connection to be activated. Interestingly, they got better reception from it than they are getting from their Telstra device; probably due to the relative locations of their and my device (the distances are > 10 m). I was satisfied with this device.
The 3864AC V3 is probably the same hardware in a different enclosure. After six weeks of use, I can state that it is an improvement. The performance of the NBN connection and WiFi are much the same. I have noted two improvements. Firstly, the 5 GHz beamforming seems to work now. Secondly, the 5 GHz auto channel selection seems to work better now.
Am I satisfied - yes. Why am I satisfied? It's free and it works for me. If it wasn't free, I wouldn't use it. I have, but don't really use or need, a fixed line phone. I don't like the constraints with which Optus has crippled its functionality.