The wrong way to look at reversibility is as black or white question (Bitcoin is irreversible, PayPal can be reversed).
It is more a sliding scale.
Every payment system in the world can be reversed. Bitcoin can be reversed by a 51% attack for example. Bank Wires can be reversed if funds were transferred in the commission of a crime. No you can't put a gun to Warren Buffet head and have him wire $1B and then say "bank wires are irreversible". Even cash can be reversed when the buyer pulls out a gun and "asks" for his cash back.
So the right way to look at the risk of reversed transaction is not YES/NO but more how HARD is it to reverse. Harder to reverse, less risk to you.
If a 6-confirm Bitcoin transaction is a 10 on the "hard to reverse scale" and PayPal is a 1 then everything else falls somewhere in between.
Everything can be reversed just some are easier than others.
The good news is Moneypaks are pretty "tough" to reverse. One can't simply claim "they didn't get the goods, or they didn't like the product" and request a chargeback like a credit card however if someone is willing to commit fraud they can be reversed by claiming the MP was stolen or it was hacked (usually the conman will claim it just "didn't work".
Theoretical call to GreenDot. Who are they going to believe?
I bought this $1,000 MP from Walmart and when I tried to use it, it says it was already used. I have the original card and receipt and everything. Please please help me GreenDot, I bought this MoneyPak to pay my bills and now I can't.
One can limit the risk by
a) only accepting MP purchased with cash.
b) only accepting MP purchased that day or relatively recently (someone selling a MP "they" bought weeks or months ago is likely reselling a MP, it probably has gone through half a dozen hands and anyone of them could have snagged the funds).
c) only accepting MP with a scan/photo of receipt (have seller write your name on the receipt).
If you did all that could you still be scammed? Maybe but the odds are pretty low. I would say it is probably an "8" (on 1-10 scale) for difficulty to reverse. Probably safer than just about anything other than Bitcoin, cash, or bankwires.
Just accepting MP from someone online with no rep selling them for 50% off (ask yourself why would they do that) however is just asking to get scammed.