The Magento platform is supported and maintained by some amazing developers, but they can only support their system. Anything you add to Magento or customize is not supported by them, so when (for example) a new version of Magento comes out (minor versions or major versions), the upgrade may break your site.
The thing is... each time an update/upgrade/patch is installed, you also have to update your installation with a compatible version of your theme and also make sure your extensions support that version. (like on Themeforest, for example -- theme developers will list out which versions of Magento are supported. Same thing applies to extension providers like Mageworx, Amasty, and others). You need matching, compatible versions on everything. So an update like v.1.9.0.1 to v1.9.0.2 could be a lot more involved than simply executing an "update" command on Magento (if only it were that easy). If you only used core Magento with the default (built in) generic template and did not make any customizations or add any extensions, then yes, it would be much easier to upgrade.
But in reality...
On a typical Magento installation, you could have 12 to 15 extensions installed. Some of them offer minor functionality changes, admin-only features, or things that rely on basic core functionality. Those are usually pretty stable between version upgrades, even when they aren't frequently updated by the developer.
Out of those 12 to 15 extensions, you may have 3 to 5 of them that involve customizations to the front-end (like an AJAX add to cart feature, or color/size swatch picker or dynamic pricing tool, etc. etc.). Those types of extensions have to be implemented to work with your specific theme, so you have to hardcode some of those changes into the theme's files and sometimes troubleshoot javascript conflicts (when you purchase an extension, developers will often help you with this -- at least until your support plan expires). These changes have to be made each time you update a theme or update an extension. In other words, everything needs to be compatible with each other. That's why you can't update Magento each time a new version is released. Everything has to be compatible or things will break. That's just how it goes.
Backups are your best friend! Lots of backups! Also, make sure you have a trustworthy Magento developer somewhere that you can reach out to when things go wrong. Google searches can only get you so far, and some things just need developers that really understand what's going on. (Developers will almost always require root access for Magento work since many operations have to be run from command line)
The reason why people recommend v1.9 over v.2.* is because the 1.* versions are much easier ("easier" is a relative term) to understand and troubleshoot (more history, more community involvement = easier to find solutions -- but don't let that fool you. There is no such thing as "easy Magento"). I mostly agree with others recommendations to try v.1.9 first. (Magento is layered like an onion. and v.1.* has fewer layers). If you locate a tutorial on how to install v1.* and compare that to "how to install v2.*", you may see what we're referring to. The main downside to v1.* is that you would have to rebuild on the v2.* platform when and if you decide to upgrade.
Really, I know it doesn't sound exciting, but you should get your feet wet with Magento first. Go ahead and set up a cheap Digital Ocean server and try installing Magento v2. See how much work it actually takes, and know that technical issues are just part of the territory. If you can successfully install Magento v.2 on your own (using your friend Google as necessary) and actually get the platform online and working (both front-end and back-end), then you might be a candidate for basic Magento maintenance in the future. Gotta clear the first major hurdle first and see if Magento is suitable for your needs.
Magento is the most powerful platform on the planet. It's capable of just about any feature or function your heart desires -- but that comes at a cost (usually a high development and resource cost) and healthy doses of patience. Try to decide if you need the complexity and overhead and capability of a Magento system, or if something easier (like WooCommerce) will do the trick. Just don't bite off more than you can chew -- development and maintenance should be manageable for you -- not a downward spiral of frustration.
Magento WILL definitely put you to the test (it puts EVERY developer to the test, regardless of skill set) -- so play with it, learn it, troubleshoot it, and see if there's a chance you want to take it further. It's a big commitment, so test the waters before dropping your client in the deep end of the pool. You have to be the one to watch over them, and save them from drowning when things go wrong. Find out how well you can swim, or be prepared to hire a trained lifeguard. And if you decide that you don't need an entire waterpark of functionality, then just stay in the shallow end of the pool (WooCommerce or similar).
