You asked how effective the sensor dust removal systems are for Nikon and Canon cameras and whether they have improved.
To answer the questions I searched the DPReview forums for mentions of sensor dust.
The search was conducted first for the entire period of all known postings and then for the last 12 months only.
The search of all postings confirmed the subjective impressions that at one time dust on sensors was a major concern for both Canon and Nikon owners, with Canon owners seemingly more affected than Nikon owners (0.94% vs 0.64%).
The search during the last 12 month period shows a dramatic four fold improvement for both Canon and Nikon owners, but with the Nikon results still slightly better (0.16% vs 0.20%).
This is striking confirmation of the subjective impressions that modern dust removal systems are greatly improved.
Methodology
DPReview was chosen because it has a large volume of postings covering a period of many years.
The following commands were used to search Google and the number of replies are the figures quoted in this study. This was done for the last 12 months and for the entire time period.
sensor dust "forum=1034" site:http://forums.dpreview.com/forums
sensor dust "forum=1039" site:http://forums.dpreview.com/forums
sensor dust "forum=1021" site:http://forums.dpreview.com/forums
"forum=1034" site:http://forums.dpreview.com/forums
"forum=1039" site:http://forums.dpreview.com/forums
"forum=1021" site:http://forums.dpreview.com/forums
sensor dust "forum=1031" site:http://forums.dpreview.com/forums
sensor dust "forum=1019" site:http://forums.dpreview.com/forums
sensor dust "forum=1032" site:http://forums.dpreview.com/forums
"forum=1031" site:http://forums.dpreview.com/forums
"forum=1019" site:http://forums.dpreview.com/forums
"forum=1032" site:http://forums.dpreview.com/forums
Caveats
The absolute number of complaints has no useful meaning since it is a self selected sample that may contain multiple mentions of the same problem. But it forms a good basis for making comparisons. The large number of postings, ca. four million for both Canon and Nikon are enough to give reasonable confidence in the comparisons.
Disclaimer
I do not use or own Canon or Nikon equipment