1.) what platforms or industries or sectors do online marketing well?
Industries that do SEO well are always going to be the tough verticals. Because they're so competitive, the barrier for entry there is quite high; if you want to compete for the big terms in the accommodation industry but your SEO is only 60% there, you're probably going to be burried on page 5. That same level of work in a local trade industry may see you toward the top of page 1 because you have no real competition.
As for platforms, all the major ones (Wordpress, Magento etc) are going to be great. There are different senarios where one may be better than the other but as a general rule, they're on a reasonably even playing field with their respective pros and cons. Where they sometimes fall apart is low quality developers making the site. The platform itself may be easy to use but if it was designed in a way that requires a 10 minute process to update your page titles, you're going to have a bad time!
2.) what platforms or industries or sectors do online marketing badly?
The only industries I've seen where most players do it badly are things like adult products, gambling and pharmaceuticals.
I'm sure there are some good ones out there but they're exceptions to the rule from what I've seen.
By "badly", I mean they use black hat tactics and aren't opposed to destroying their competitor's link profile to move up another spot.
3.) is doing online marketing well or bad a function of the platform, industry, size of business, budget, knowledge, tools?
Being able to observe great SEO is a function of strong competition. _Doing _strong SEO is really a combination of all factors you mentioned. Really, you should have a platform that's easy to work with and offers good site speed, an industry suitable for your resources (if you're a sole operator doing your own SEO, competing with someone like Trip Advisor is possible... but probably not the best idea.). More budget and knowledge generally means you can work faster and climb those results at a more impressive pace.
Also there are many industries .. .who may to varying degrees rely on online marketing to drive leads and keep the phone ringing.
About the only industries that need SEO are those who exist almost entirely online. If you're running a small ecommerce store and nobody can find you online, it's going to be a struggle. Having said that, almost any business can benefit greatly from good SEO.