This is already documented in the release notes but I wanted to call out that in Exchange 2013 you must use local powershell to run the cmdlets to install transport agents in Front End Transport (FET). This is because when a remote session is used the cmdlet is executed on the backend server. When you include the assembly path the cmdlet executing on the back end attempts to load the assembly at the location you specified. When the assembly isn't there, it can't load it and it fails. For example, if I wanted to install a transport agent on the 20013 CAS and I placed the assembly at C:\transportagents\mytransportagent.dll, the cmdlet, executing on the backend, would look for it on the backend local drive instead of on the local drive on the CAS. It wouldn't find it and issue the following error:
The transport Agent assembly file "C:\transportagents\mytransportagent.dll' doesn't exist. Parameter Name: AssemblyPath."
To get this to work you must use local powershell so that the cmdlet executes on the local machine. Now, I know you are thinking to yourself "Dave - you told us that this isn't supported!". I know I did, however in this case you must use local powershell and the product team has agreed to support these cmdlets run in this way.