I Do NOT Agree
As you know, I deeply respect your skills and your opinion....with just a couple of exceptions (like the need to separate completely house and start battery banks).
However, I do not agree at all with your characterization of the Xantrex EchoCharge device.
I have one on my own boat, and have carefully watched it's operation over the past five years or so. Including most every day. Including today.
My Victron Multi-Plus was in the "repeated absorption" mode, putting 14.5 VDC into the house batteries. The charge on the start battery, occasioned by the EchoCharge, was 13.6 VDC....a proper float voltage.
I have watched the EchoCharge when in the equalizing mode...over 15.5V going into the house batteries. The EchoCharge never put more than about 13.6 volts into the start battery.
On my boat, and on a number of client boats on which I have installed the EchoCharge, I find it absolutely flawless. It works perfectly...does not overcharge the start battery, and just sits there and does it's job as it's supposed to.
Xantrex, by the way, bought this device from another company and, thankfully, has resisted the temptation to change it. As far as I know, anyway. It's one of the very few Xantrex devices that I can and do recommend and install.
By the way, I completely agree re: the instructions: they need to make the connection info more clear.
Bottom line: Yes, the EchoCharge is a "voltage-follower device", but it is a VRD with smarts. It won't harm your start battery and, if properly connected (which is VERY VERY easy) it will just do its thing and you don't have to worry anymore about your start battery.