It may sound nitpicky, but I think that description leaves something to be desired in terms of presenting the “correct” mental model. First, there is no such thing as “a” bitcoin, as I am sure the author would agree. Speaking of spending or moving bitcoins perpetuates the notion of bitcoins as “things”. It might be preferable to say that you are spending or moving “units of the bitcoin protocol”. There is something similar going on here with dollars. The dollars in your bank account aren’t things either, they are units of demand or claim on a currency. The fact that printed dollars have serial numbers tends to confuse this notion. Treating something as a “thing’ which is not a thing is sometimes referred to as the reification fallacy.
Hasta la fecha (Agosto del 2016), las sidechains sobre Bitcoin no son más que algo teórico. Una implementación de este tipo requeriría de un cambio en el código Bitcoin (hay miembros de la comunidad Bitcoin con gran prestigio, como es el caso de Peter Todd, que argumentan que una sidechain, tal y como la describe Blockstream en su paper, no podrían llevarse a la práctica en Bitcoin sin hacer un gran cambio, hard fork, en Bitcoin). En el mismo paper de blockstream se reconoce que una implementación de este tipo, la cual su teoría es simple pero su implementación compleja, se enfrenta a problemas que no está del todo claro que puedan solventarse (y no todos son de tipo técnico).

In September 2015, the first peer-reviewed academic journal dedicated to cryptocurrency and blockchain technology research, Ledger, was announced. The inaugural issue was published in December 2016.[91] The journal covers aspects of mathematics, computer science, engineering, law, economics and philosophy that relate to cryptocurrencies such as bitcoin.[92][93]

“Not only is decentralization, open protocols, open source, collaborative development and living in the wild a feature of Bitcoin, that’s the whole point. And if you take a permissioned ledger and say, that’s all nice, we like the database part of it, can we have it without the open decentralized P2P [peer-to-peer] open source non-controlled distributed nature of it, well you just threw out the baby with the bathwater.” 

Further, despite sidechains being independent of each other, they are responsible for their individual security and need the requisite mining power to remain secure. Bitcoin’s blockchain has sufficient PoW mining power to remain secure even from the most coordinated of attacks, but many more nascent sidechains lack the necessary network effects and mining power to guarantee security to users.
Security: RSK´s blockchain is secured by merge-mining, which means that they can achieve the same security as Bitcoin in terms of double-spend prevention and settlement finality. The 2way peg security will first rely in a federation holding custody of bitcoins, and later switch to an automatic peg, when the community accepts the security trade-offs of the automatic peg.
Always there is a balance in nature, even in blockchains. If you want to have extra features, you need to make a sacrifice from your current features. For example to have high speed and volume; you need to give some from your security & immutability by doing consensus with smaller groups or you need to use different methods in consensus like POS / PBFT. (Proof of Stake / Practical Byzantine Fault Tolerance)
At Iryo, we consider databases and blockchains that are not opened to the public to be insecure they, can easily be altered by the business running it, at their discretion and it goes against the ethos of the open and transparent cryptocurrency space. Designed to keep public out and introducing “trusted” middlemen, private chains forget that trusted third parties are security holes.
“The reason why you put up private blockchains is potentially because you want to have control over the participants in the blockchain. So as we have banks and financial institutions, who have to worry heavily about regulations, they can’t use the public blockchains right now because they are open and permission-free, and anyone can participate, and that’s contradictory to the regulations to which they must abide.
What is the difference between a public blockchain and a private blockchain? Does it matter? Which is better? Gallactic believes that currently there are pros and cons between both Private and Public Blockchains, but time and “convergence”, a term that is gaining prominence in the Blockchain Industry, is clearly showing that the lines between these categories, once clear, are starting to fade.