Like any weapon, when you start modifying the original design (G17), you tend to lose a certain degree of reliability--the G19 (and to a lesser extent the G26) are prime examples.
That being said, the 357 SIG really offers no feed reliability advantage over the 9x19 (which is BTW, a tapered design), and in fact, as previously noted, the bottle-neck design (and particularly as implemented by SIG) has its own set of reliability problems.
The first and foremost, as noted by PAT and others, is a tendency to nose-dive. The tendency can be exacerbated by several factors including mildly weakened magazine springs (which would still work with any other cartridge) and dirty magazines, (or dirty, dusty, sandy conditions--no matter how assiduously you maintain your magazines--probably PAT's problem in the Yukon Delta). Other reliability issues with the 357 SIG include: (1) lack of case neck tension (particularly a design problem of the 357 SIG--the case neck is just plain too short); (3) the need to "headspace" in two places on the case mouth and on the case shoulder (again, a design problem--bottle-neck cartridges should headspace on the shoulder and not the case mouth); (4) that funnel shaped chamber which is so good at "catching" .36 calibre bullets is also well suited for catching debris which remains in the chamber instead of falling out the barrel--the old .40+ calibre in but .36- calibre out trick.
The problem with the 357 SIG (and the .40 S&W) is they are both compromise rounds--an attempt to force performance into a 9x19 size platform instead of going to the logical step and designing an intermediate size weapon (between the 9x19 size platform and the 10mm/.45 ACP size platform)--much like the S&W L-frame and Ruger GP100 which fall between the K-frame and N-frame (or Rehawk) size.
The good news is that finally, the 9x19 is being optimized to provide performance which is at the very least equal to the 357 SIG while still maintaining all the advantages of 9x19 (low muzzle flip/blast, high-capacity and feed-reliability). The 357 SIG may have been a good round in its time, but it has definitely been OBE'd (overcome by events).