All I know is that I have Philips Hue lights, Belkin Wemo switches, and a Nest thermostat and none of them connect to HomeKit. For the Hue I have to buy a new bridge, Belkin says that they can't support it without completely new hardware[1], and Nest is completely mum on the issue but presumably there's no hope of support in the near future.
These devices all have active developer communities and good API/interaction. I plugged my Amazon Echo in and it immediately detected all three and allowed me to control them seamlessly. I don't know what the specific issue is but it sure seems like Apple is the problem.
[1]: http://www.belkin.com/us/support-article?articleNum=187953