It was never intended for this mod to replace screen recorders. The purpose of the mod is more to allow better cinematics - timelapses, slow/fast motion replays (like the name), and anything that would normally require having a second account recording from spectator mode.
I believe server-specific resource packs are either planned or already coded in, but inventories are highly unlikely; especially because the mod can only see information sent by the server directly to the player who is "recording" the replay.
I agree on what you are saying, but there is just no way of rendering a hotbar in a VR/Equirectangular video using screen recorders, this mod could have the possibility of doing it as i explained in another thread here:
https://www.replaymod.com/forum/thread/865 (i'm sorry for not searching before posting, i'll do that next time)
I think this would allow a lot of cool things (VR Let's Play), but i'm not gonna stress too much on this
Right now the mod has only access to what the player recording is receiving from the server, but it's still a forge mod, it can have access to everything, the code is not there right now