According to eMarketer, 53.9% of all U.S. e-commerce sales will be made via mobile devices by 2021. Among all of the mobile e-commerce apps, Amazon ($NASDAQ:AMZN) is the most popular by a long shot, according to Verto Analytics. In fact, Amazon's monthly reach is 86% compared to 53% for second-place Walmart.

Amazon CEO Jeff Bezos points to a positive cusotmer experience as Amazon's biggest advantage. From the ordering process, to payment, to delivery, to follow up, few e-commerce outfits can match Amazon when it comes to making sure customers are happy with the experience.

But things aren't perfect in the browse-to-buy equation. While Amazon is busy figuring out the last mile, testing drone delivery and promising 1-day delivery to extant customers, problems remain in the mobile shopping experience.

According to app review data for Amazon's leading shopping apps on both the Apple App Store and the Google Android Play Store, the Amazon e-commerce app is far from perfect.

On the Apple App Store, "Amazon - Shopping made easy" is listed as the #1 app in Shopping by downloads. However, as of today, it scores just a 3.3 out of 5 stars. Users complain of a crashing app, a buggy interface, and problems with the shopping cart. 

At the Google Play store,  "Amazon Shopping" fares a bit better, with a consistent 4.3 rating for its latest version across close to 1.1 million reviews. That said, it isn't free of bugs, especially as of late April when the latest Amazon app version caused usability issues for several customers.

To be fair to Amazon, the company iterates and releases app udates. On the Apple App Store alone, the Amazon has released 42 version updates since mid-2017 alone, and average reviews for each version have improved generally with each iteration.

That all said, it's a curious blindspot for Amazon when it comes to improving customer experience, especially as it relates to a product that's so close to the company's chest. And, perhaps even more importantly, a product that is soon to be the most-common touchpoint customers have with the brand.