Mobile video ads have become a huge boon for some companies, as spending for the format is set to reach $2.62 billion this year alone, accounting for about a third of the overall $7.7 billion that advertisers will spend on digital video ads. eMarketer reported these numbers, estimating that they’ll grow even bigger to nearly half (47.7 percent) of total digital video ad dollars by 2019.

Consumers are watching a lot more video on the market, with an estimated 105 million U.S. smartphone users watching at least one ad on a monthly basis through one of their devices — a 13.9 percent increase from the previous year. Tablets is right up there as well, with more than 100 million users watching a video once a month, representing more than half of all tablet users.

That said, there are concerns about the format, according to The Video Ink. While numbers may seem high on the front end, it’s only expected to account for more than a third of overall total digital video ad spend for the year, even with more creativity coming from publishers and video creators alike.

“Mobile video ad spend is experiencing rapid growth, thanks to larger audiences of video viewers and growing time spent on smartphones and tablets,” said Jeremy Kressman, analyst for eMarketer and author of the report (which can be found here, via a subscription). “But changes with the format are causing growing pains for the industry.”

There are a number of formats to be used with video advertising that companies have to consider, such as in-stream advertising, in-banner, in-app and interactive videos, among others, making it hard them to commit to a particular format – at least, until they see a “clear winner” that would pay off in the long run.

Mobile video still has time to iron out the kinks, however. “As consumers are becoming increasingly agnostic about the device they use to watch video, some advertisers are finding the most valuable strategy to be buying mobile video ads as part of a broader cross-device campaigns on television and digital video,” added Kressmann.