I would consider first two relatively simple two implement from VMS side and Recording Control I find challenging for the both sides (camera, VMS)
Profile G is really about first two. Recording control is mostly intentionally moved to conditional part of ProfileG because of it's complexity.
If we consider only mandatory features, Onvif replay streaming format has some extensions to standard RTSP protocol. This means that you have to hack your rtsp library to work properly and implement some things. And it is not as simple as receiving live stream from camera, for that you can use freely available standard libraries. But this work have to be done from camera side also, so it does not answer your primary question.
Most challenging part I see is to integrate this feature to legacy VMS design. And this task is really depend on internals of specific VMS.
Nevertheless we successfully implemented ProfileG client for Intellect and Next. This two products are very different inside.
Recording Control gives ability to manage space of device onboard media and ability to configure recording job.
Oversimplifying things, "managing space" means that if you have 16 channel NVR, you can create separate recording archive for each camera with different size.
With recording jobs - ONVIF gives ability to start or stop constant recording with specified fps, resolution, video codec. Limitations: only constant recording, there is no way to configure recording based on onboard motion detector events.
Related, does Axxon implement the recording control.
No, we don't have it currently. We are eager to implement recording jobs control - ability to start stop recording. In fact we have most of the work done for that, but lack of proper support from device side is the main problem for us.
Considering the fact that Axis has implemented G in numerous cameras, as well as the fact that they are an active member in the G committee, not to mention the fact that they started and chair ONVIF itself, one would expect that their "top 5" VMS would implement G matter of factly.
I do not like it and do not think they should use that. Related, I think ONVIF should not allow it, not only because it could mislead but because the likelihood of 'relevant parts' actually working is much lower.
We develop onvif G cloud storage solution I am looking for list of cameras that support onvif G - at the moment we support hikvision dahua pannasonic but I would like to add more brands Dov Avnon the Netherlands
I want to set appropriate expectations. Because Milestone rarely promotes the ONVIF bridge and its a niche thing to begin with, it's low on our list of priorities. If Milestone or you feels that it really works now, that would makes us moderately more interested in testing. I don't want to spend resources testing something that was broken and that a company does not promote, because very few will then benefit from it.