You'd think so, but it's not that easy.

Even the big service providers, like Dropbox and Microsoft have run into issues of scale that result in price increases or reduced storage quotas, later in the product cycle, forced onto existing customers. Typically a result of poor early planning or lack of insight into actual resources required per user.

Or bait and switch to grow fast quickly.

Dropbox, recently (quietly) raised prices on their Business plan, forcing existing Business customers to choose between a fixed storage space quota or a price increase. The decision will be forced on users on renewal in 2018, which is why we haven't heard much about this yet.
https://9to5mac.com/2017/01/30/dropbox-smart-sync-and-paper/

Microsoft had to do similar a year ago. Forcing all users off their unlimited plan, and reducing storage quotas for free users. It was painful.
http://www.digitaltrends.com/computing/microsoft-onedrive-un...

At the same time you've got a race to the bottom from big companies targeting the consumer market. These providers most likely have no hope of generating revenue directly from the cloud storage itself, and instead use cloud storage as a loss leader for other channels and products.

For example, Google integrating free "unlimited" photo storage into their products, Apple iCloud integration and significant price cuts. If these loss leading experiments fail there will be another round of forced price increases or reduced storage quotas. Only time will tell.

So far it's a good bet against "Unlimited" free services:

Near-Unlimited Cloud Storage Service Copy.com Is Shutting Down:
http://lifehacker.com/near-unlimited-cloud-storage-service-c...

Bitcasa kills infinite cloud storage plans, increases pricing as much as 10x
http://www.geek.com/apps/bitcasa-kills-infinite-cloud-storag...

Then you've got Amazon cloud drive, offering unlimited Storage, but only for non-commercial personal use, with all sorts of restrictions. Sole practitioners and small businesses not welcome.

Using Your Files with the Services. You may use the Services only to store, retrieve, manage, organize, and access Your Files for personal, non-commercial purposes using the features and functionality we make available. You may not use the Services to store, transfer, or distribute content of or on behalf of third parties, to operate your own file storage application or service, to operate a photography business or other commercial service ...

And this open-ended goodie:

The Services are offered in the United States. We may restrict access from other locations. There may be limits on the types of content you can store and share using the Services, such as file types we do not support, and on the number or type of devices you can use to access the Services. We may impose other restrictions on use of the Services.

https://www.amazon.com/cd/tou??ref_=cd_unlimited_tou

And finally most consumer grade cloud providers offer little protection in terms of encryption. Google, Amazon and Microsoft might get big brother on your data. You might even have real people looking at your files for whatever reason.

Our Use of Your Files. We may use, access, and retain Your Files in order to provide the Services to you, enforce the terms of the Agreement, and improve our services, and you give us all permissions we need to do so. These permissions include, for example, the rights to copy Your Files, modify Your Files to enable access in different formats, use information about Your Files to organize them on your behalf, and access Your Files to provide technical support
https://www.amazon.com/cd/tou??ref_=cd_unlimited_tou

Our automated systems analyze your content (including emails) to provide you personally relevant product features, such as customized search results, tailored advertising, and spam and malware detection. This analysis occurs as the content is sent, received, and when it is stored.
https://www.google.com/intl/en/policies/terms/

Big brother with good intentions:

Microsoft Anti-Porn Workers Sue Over PTSD - ex-employees of the company’s online safety team say they had to watch horrific online videos of child abuse, bestiality, and murders—and that Microsoft ignored their PTSD.
http://www.thedailybeast.com/articles/2017/01/11/microsoft-a...

Finally, as a few others in this thread have stated, the underlying technology behind syncing and sharing is quite complicated. Cross-platform compatibility and maintaining backwards compatibility with older operating systems is mind boggling difficult.

In this regard in might be better to choose a cloud provider that offers end-to-end encryption, and has been in business for more than a few years (to ensure they've got the business model worked out).

Here's a few that come to mind:

https://www.spideroak.com
https://www.sync.com
https://www.tresorit.com