I wouldn't say they were tougher. The Legions conquered entire star systems and could cleanse a planet with little support from the guard. The Chapters on the other hand need alot of support to go to war, using vast amounts of Imperial Guard and/or other allies to take the main brunt of battles while they perform specfic tasks. The pyscho conditioning is a bad thing to me, it just makes them less human and so much more two dimesnional in the majority of cases. I wouldn't say that they had less inner strength either, i've not read any instance in the HH series so far of the marines giving up or running out of steam, the living emperor AND their primarchs drives them forward. Just look at Istvaan III.
And on the point of achieving things without a entire legion backing them up, goes back to the point above that instead of a couple thousand astartes they now have millions of guard troops to fulfil the role instead. There havent really been any instances in the HH to compare them to 40k marines who are now facing chaos deamons, tyranids etc all the time.