I think that by their very nature, Synths will prove to be extremely challenging opponents. Firstly, they appear to be highly adaptable to combat situations, even continuing to fight with damage to their face, as seen with the Synth Assaulter who takes two direct shots that would have instantly killed any human enemy. Also, they seem more than capable even with broken limbs and other structural defects, which certainly does give a distinct 'terminator' feel. And their Cryo weapons would appear to use the equivalent of frost damage, as seen in Skyrim, and if so would likely slow down and eventually stop your rate of movement as well as damage your health.
But what I find most interesting about these units is the way they look. They are very different in appearance to the androids we see in Fallout 3, and they look far more like actual 'killing machines'. But not only this, their appearance could suggest an early production model designed by the Institute that were perhaps listed for termination. The first one we see in the E3 demo has what looks like fire damage to its face and parts of it's body. And the second one also has other structural damage. So perhaps they were sent to a 'disposal site', but some of them either did not die, or managed to escape and have now gone rogue.
If this is the case, and these formidable enemies are actually rejects, then what else have the Institute created that would be even more terrifying?