I run my own little ad blocking DNS service and the thing I'm probably most concerned about is websites/publishers/etc. deciding to serve their ad and user tracking assets from their own domains. Though most would probably need—or choose—to use subdomains and be easily blockable all over again.
Same here, I can think of a million ways to circumvent DNS blocking. The good thing is, most sites are not gonna adopt them very quickly. Besides, DNS blocking would have value even if all sites started doing that: there's plenty of malware/phishing domains to be blocked and I don't think those have better alternatives!