What is cross platform advertising?

Mitch Turck (2015) 
Cross-platform, also known as cross-media, is a reference to the state of your advertising spanning multiple media types for example, mobile and paid search. I say "reference" because the phrase out of context doesn't have useful meaning. Anyone with a decent budget advertises across multiple platforms/media types. When you hear "cross-platform", you're usually hearing it as a reference to the measurement being conducted on such advertising, in an effort to attribute one result back to your holistic advertising efforts. Traditionally, the sale would have been attributed to the ad that generated it.

What are the pros and cons

Pros: Optimize and improve your advertising. They can be used as a market research tool to determine which advertising platforms are the best and re the most effective in reaching your target audience. It is also a proven fact buyers respond to repeated exposure and it doesn’t leave anyone out since there is not typically a select target audience.

(Robert Morello)

Cons: Loss of Focus: The more different types of marketing channels your small business uses to get the word out, the less focused each of your individual marketing campaigns may become.

Mixed Messages: It is often difficult for businesses who engage in campaigns that utilize multiple marketing channels to maintain a constant and consistent marketing message throughout.. Consistency is the key to establishing brand and product recognition and prestige, and to developing loyalty among the consumer base.

Increased Expense: The more variation there is in your marketing channels, the more staff you will need to run them. In addition to staffing concerns, varied marketing channels often cost more to develop, launch and run.

Scheduling: The scheduling involved with campaigns that are distributed through multiple marketing channels is always a challenge. Each type of campaign and each form of promotion typically has its own development, completion and distribution schedule.

To conclude cross platform or cross media market is an effective way of getting an audience to buy or check out your product based on the fact that it advertising it across many platforms will get it out to all people of any age however if not done correctly it may lose you customers since what your advertising and what your selling may be seen as two different things which thusly makes mixed messages therefore turning customers off of your product.