@Deane. Insteon and X10 are different protocols and frequencies and X10 is not transmitted with Insteon period. However the powerline frequencies are close enough some phase couplers will work for both.
ISY 994i with a PLM can transmit and understand Insteon and X10 but unless yo transmit an X10 command none will exist.
Insteon is a dual band self synchronising mesh network. All newer powerline modules transmit and receive via RF and powerline. Battery devices only use RF to the nearest device and get converted to both.
Signals are echoed by all devices in a synchronised transmission from powerline crossings so that more devices strengthen the signals.
Hops are controlled by the protocol settings in devices and some report reliable control over 500 feet away in barns.
Retries are also set in protocol settings of each device.
Transmissions are acknowledged for reliable communications.
Device status changes are reported immediately.
The Insteon Hub is the bottom end of the HA definition. ISY994i is the top of the line for automation.
After reading reports from other technologies I have realised that I am very glad I went with the ISY994i and Insteon.
Analogue inputs, door locks, and noise makers are scarce in Insteon but Zwave can fill these gaps with the optional plug in Zwave hardware for the ISY 994i.
With it's REST interface ISY994i can accepts controls and variable values from any other PC, RPi or other controller.
With it's optional Network Resource module ISY994i can easily control Hue bulbs and Nest thermostats.
The ISY994i draws about 2-3 watts and is the size of a cigarette package.
With it's currently being developed PolyGlot node interface a RPi can act as a distributed intelligence box to ISY so that users can write code to interface with any device hardware the RPi can talk to with it's options. Hue bulbs now have a drop in node module that is included so far. Many others are coming to an available library soon.
This new firmware will also allow two ISYs to act as one so that your cottage ISY could act like it was part of your local ISY at home.
With the proper scene setup using a MS and Lamp control with Insteon no delay can be detected. After the scene is implanted in devices, ISY could be turned off and controls can still function without it. Of course you would lose monitoring, notifications, and logic without ISY.
With Insteon scenes, 200 devices could all be turned on simultaneously. ISY994 then would follow up to see if every device operated OK.