One thing that has to be considered and can become an issue with under volting is that multiple transistors make up circuits. Some times these transistors work together and some times they fight each other. Even two transistors in close proximity to each other on the same chip will be slightly different due to variation. As voltage decreases for some circuits this variation between transistors gets exposed as one transistor approaches the treshold voltage while the other may not, while at higher voltages this slight variation may not come into play as strongly, leading to outright failures or stability problems that get exposed at particular frequencies.