Not the replier, but:
1. Cost, firmware upgrades were a pain, a handful of the devices seemed to have rotted away from a rubbery plastic into a sticky mess. Anyone with Amstrad devices in the 80's and 90's might know what I'm talking about here.
2. RPS server allows a fresh out of the box Yealink to dial home as soon as it's plugged in. You would need custom firmware files from a supplier to achieve this otherwise.
3. RPS server doesn't do the bulk of the config, FusionPBX still does that.
4. Not a big fan about the number of devices they have out...but that seems to be all companies. More SKUs is just frustrating to support, I constantly have to Google how many DSS keys aa T43U or a T31P has.
5. There is a Shared Call Appearance mode for Yealinks, which I believe modifies how the line button works slightly, I never use it as it's a bit of a legacy feature from those old key system PBXs. The SIP spec has no problems registering multiple handsets to the same extension, and most people do this naturally with a handset, a desktop app and a mobile app. Emulating a key system like the olden days is probably better done with Parks and BLFs for the parks, though FreeSWITCH/FusionPBX/Yealink all seem to support SCA if that's what you are after. Worth searching this forum for people's experience with it.