Hi
The interface had Nothing to do with how powerful it is for mixing. The computers power (CPU RAM) is what determines how complex the mixes can be and NOT the interface. That person doesn"t know what they are talking about
The interface you are interested in will most definately be enough and what you need to look at is how many in and outs you need and if you are gonna use lots of UA plugins, that can be the roadblock as you would need a quad interface from UA if you wanna use lots of UA plugins.
as far as how many plugins you have that are not UA too use in a mix is not at all dictated by any interface so :
If you are going to use lots of UA plugins you might consider the twin duo interface as it has a little more horsepower for,plugins but the exact same sound quality as the larger interfaces, and if you are gonna use 12-15 instances of UA plugins and the rest your other plugins you will be all good.
UA has a comparison chart on how much DSP each of their plugins use from the interface so check that out before making the purchase as it varies quite a lot from interface to interface.
And just to be clear : the interface in this case has nothing to do with how big your mixes can be, that is ONLY dictated by your computer power and RAM aswell as having the correct requirements with Hard drives etc.
Please read up on UA website yourself instead of listening to people who haven't got a clue of what they are talking about so you yourself have all the info you need to make the right decisions going forward
Here is the chart of how many instances of plugins ypu can use on each of the UA interfaces:
http://www.uaudio.com/support/uad/co...nce-chart.html
Best of Luck
Christopher