I used a couple of third party chargers for my SP1 that came from China that I got off
ebay.
One was OK, but the other melted.
I'm glad I happened to be looking at my SP1 as I was working at the desk because I saw it start to change shape and when I grabbed it, it was really hot.
I dread to think what would have happened if I was not there and it kept melting and maybe damaged the SP1.
The other third party charger is OK though... so, it's kind of a lucky dip.
I haven't bought a 2nd charger for my SP3 yet, but if I do, I think I'll just pay the extra for a MS one rather than risk it.
I think one problem is that these chargers are not like the standard barrel chargers of most laptops that just have a positive and negative wire going to the charger. Those things can be replaced by just about any other barrel plug that has the same size, polarity, wattage and amps. Also, just about anyone can easily reproduce them.
The SP1 SP2 and SP3 chargers have more connectors and if the person making them cuts corners or just ignores some aspect, only concentrating on connecting the power wires, then there is more potential for something to either short or not function as it should.
Like I said, my other after market charger for the SP1 has been fine, so you might be OK, but I'd just make sure you don't leave it charging unattended.