I haven't used Kunversion so I can't comment on that platform. However, I have used Tiger, Boomtown, and Market Leader. Tiger Leads, at least the last time I used it, did not have an email drip system like Boomtown and Market Leader. Tiger doesn't have a Craigslist Posting tool, nor does it generate copy and paste ads either. All three offer PPC services, but Market Leader has a most extensive source of quality lead generation sites (i.e. HouseValues.com, JustListed.com, etc). Tiger has some homegrown ones as well, but they don't get the traffic like Market Leader's properties. Ultimately I think the real questions is which one converts the best. Just speaking from my own personal experience, Boomtown converts leads-to-sales better than Tiger or Market Leader.
Here are a couple of reasons that come to mind;
1. It has a low barrier to entry in terms of budget and setup
2. You can edit and maintain your own site with a little education
3. Integrates with all your social media marketing, and can be used to create custom tabs on FB.
4. Very Google friendly
5. Has thousands of free or very low cost plugins that you can use to enhance your site (i.e. you don't have to be a programmer to integrate rich features)
Perhaps spam filters at the server level are catching the emails. Does your domain DNS have a SPF record?
You can use semrush.com and get a good idea by analyzing sites like housevalues.com
This type of service has been around for a while now, but it hasn't gained much popularity (www.mountainofagents.com)