I have a Shimano mtb derailleur on my custom road bike. The low gear combo is 1:1, a 34-34. I did this in 2011, before road cassettes had lower gears than 27 or 28. I have never had an issue with it. My friend has the SRAM version of it on her 2010 Ruby.
I have the 11-32 road gearing on my Trek and it is fine, but I will go to the 34 next year when it comes out.
Anything to save my aging parts.