One thing I have noticed is people don't pay attention to the font size in their ads or emails. Having a very small font makes it nearly impossible to read on a mobile phone. I have found through some experimentation a Arial size 14 font is the most readable on all devices.
The main problem with using an HTML based ad or email is some email programs won't let any HTML display and only show the text part (our security consciousness about bad HTML in emails and such) or won't let the pix or other stuff through either. The email and ads then look like crap or the most important parts are missing. In these cases, if you have all your info in a nice graphic JPEG and no other text - they see a bank email - DELETE.
The only solution is to make sure the text of the ad copy or email stands on its own in case that is all the customer sees. Many seem to rely on the colors and shaders to do the work on poor copy. I use them too but for emphasis and not the message.
The same happens when you use tracking pixels and the mail program won't let it through. Gives way wrong numbers in your counts. Even if there was a 100% open rate it wouldn't show it.
THEN you have the problems your "security suite" may inject too. Even if you have your email set to show HTML, your security program may still not let it or it won't let it send back the hit data through the firewall. You tend to see marketer start whining about the low open rates all of a sudden right after a security suite updates, email programs have a security update, or the OS does...hmmm.
There is no perfect solution and you just have to keep track of your results to know what works best for you, your ads, and your customers.