I'm asking about the sentiment that ECN demo accounts are not representative of what a live account could of have been.
Is there something negative that does not happen on ECN demo accounts?
Demo account price feeds can and often do in fact differ from the live feed from a broker.
Demo accounts are simply that, they're demo accounts... the results should NEVER be trusted to perform identical in live trading.
As Professional4X started, ECN Demo is still Demo: the 'negative' aspect you asked is the slippage and latency you will experience on Live but never on Demo.
For example, the scalping EAs Robotron and RedEye I personally tested on Demo and had the expectation of profitable performances. However, running on ECN Live I did not have the same performance, due to slippage and latency during order execution.
This can be expected of tick scalpers such as Waygrow EA, MillionDollarPips Ea and many others.
Also, when trading large Lots you will experience this even if you are not scalping. I was trading 20 Lots per trade and often suffering some slippage of price when entering.