The second article you mentioned is more critical of Magento based on the experience of the reviewer who describes themselves as a "fairly technical end-user" but at the time it is written the author was at an early stage with Opencart:
...now thinking to move my overly-robust Magento store over to OpenCart. My biggest complaint? I haven't had a lot of time to really see how buggy OpenCart is, and have not used it day in and day out to test features
Here is my reply to an older thread
[webmasterworld.com] about OpenCart: >Is it secure?
Unfortunately the core developers of the project have not yet adopted secure coding practices. There are (stlll) unpatched vulnerabilities: [websynn.com
...] This official response from May 2012 indicates that this is unlikely to change under the current leadership: [forum.opencart.com
...] >is it stable?
OpenCart are not as careful in their testing of new releases in the way that PrestaShop or Magento are. When a new release comes out your custom templates are easily broken.
(There also tends to be a new release a couple of days after any major release correcting issues they didn't test for. I am sorry to report that many times these have been obvious cases that should have been tested for.) >is it enterprise level?
OpenCart does have a healthy paid for extensions store so you can usually buy the features you need.
These modern AJAX carts mentioned (PrestaShop, OpenCart, Magento) are based on PHP 5 which has better overall stability and security than the older non-AJAX carts (osCommerce, Zen Cart, etc) which came from earlier less secure PHP versions. However, it depends on the individual project and as I say unfortunately Opencart has not adopted secure coding practices. PrestaShop and Magento have.
(Of course AJAX has more overhead that non-AJAX carts, but those non-AJAX carts have a far poorer end-user experience and really are for legacy use only these days.)
OpenCart needs a lot less server power than Magento to run, but they are not really equivalent. PrestaShop and OpenCart are more equivalent in hosting etc requirements - in speed of raw performance OpenCart may have a slight edge "out of the box" but that's not the only consideration..