What makes DSSS a more robust transmission technique compared to FHSS?
I think that claim, taken at face value [sound byte'd], is somewhat dubious. Such a claim depends on the application. I also disagree that one inherently more robust in terms of interference immunity. If you are operating in a fixed channel using DSSS and you have a loud interference source in the same channel (could be fixed frequency - but higher power than you) you will loose a higher % of your packets than if you were using a FHSS system that utilized the entire band. But if you are using an FHSS system in an area where a lot of your neighbors are also using FHSS and have the same general set of hop channels then you would be in trouble. I've run into both of these situations. The first was using 900MHz ISM DSSS radios in an area where a local internet provider was using louder licensed (somehow) radios in the same channel as me (they were also using DSSS) - that channel was relatively saturated/highly utilized. The latter case was when using a FHSS radio that only had a small set of channels that it actually hopped to - so it didn't utilize the entire band. If there were other broadcasters in the same part of the band it was in it would loose a higher percentage of packets. In particular - I was jamming myself (or rather some installations of the product I was supporting were installed close together and were jamming each-other).

A poor implementation of either can lead to jamming / excessive packet loss. Also, there is no reason why you can't use a combination of the two together to more uniformly utilize the spectrum. Essentially, utilize DSSS at each FH channel.

There are some obvious decisions you should make when designing a system. For example, if you want to operate in the 2.4GHz ISM band you should pick channels between the WIFI channels if you know your device may operate in an area with multiple wifi networks (e.g. avoid picking something right in the middle of channels 1, 6, and 11 - pick frequencies between channels 2-5, 7-10, 12-14).

This article actually says that DSSS systems do WORSE with multipath / delay spread in large areas. But in enclosed small areas they do alright.
"We shall also conclude that for long distances, point-to-multipoint topologies in reflective 
environments such as cellular deployments in a city, DSSS has no chance to survive, leaving 
FHSS the absolute winner, based on its famous multipath resistance."
http://sorin-schwartz.com/white_papers/fhvsds.pdf