These Myson Heaters - the type that are connected to the Heating System - Basically they have a `Heat Exchanger` - Heated by the Heating System - and a `Variable Speed Fan` - controlled by Switches - which `Blows` out the Heat at various speeds - You will know this - but for `others`.
If the Heating System is of the `Sealed System` type - You should check that the MAXIMUM Pressure within the System - When `Hot` does not exceed the Maximum Pressure Rating of the Myson Unit.
The `Pull Forward` facility is achieved by the use of `Flexible -`Braided Stainless Steel Reinforced` - Connectors` - SIMILAR in appearance to the Flexible Tap Connectors - BUT Please ENSURE that they are `Rated` for Heating System Pipework / Pressures / Temperatures, Flexible Tap Connectors are NOT `Rated for Heating System Use - Even if the `Pressure` and Temperature` Ratings SEEM to be `O.K.` - As they would NOT be `Rated` for `Continuous Flow` at those Pressures and Temperatures.
IF any of these are `Supplied` with the Heaters ? - they are likely to be of the `Short` variety - used on Installation to Prevent Vibration Noise being transmitted through the System Pipework - Not really to allow the `Pull Forward` that You mention.
Unless You are `Wiring` the Myson Units yourself - you should mention to the Electrician to Wire the Units when you have them `Pulled Forward` - Otherwise they might restrict the movement with a `Short` cable - Just a `Tip`.
These should ALWAYS be Installed with Isolating Valves - chosen to be able to use `Partially Open` in order to `Regulate`/`Balance` the Flow & Return to the Myson Unit - Not the `Ball Type` Lever Isolating Valves,
these Isolating Valves can obviously be used IF the Unit ever needs to be replaced - so as not to have to Drain Down the Heating System.
I hope that this is of some help - My U.S. Colleagues are better placed to answer your Question about whether the Myson Heaters are as `Good` as the other Manufacturers that you mentioned [As I am U.K. based].
Some of what I wrote here is for the benefit of other `readers` - who are not aware of these types of Heaters.