Changing to WPC style parts is not an upgrade. It's a hack designed to replace previously unavailable parts.

Beware of the PBL parts, they aren't 100% correct and need some modifications to work with the proper stroke. Also, at least on the ones I tried a couple years ago, the baseplate gets magnetized very easily which has adverse effects on the flipper (usually it will stick up because of residual magnetism).

Unfortunately other people just don't make the parts.... I scavenged a lot of parts off of junker playfields years ago so I have a stock to choose from, (ah, the days of picking up $10 populated playfields from shows, I miss those....). The pop bumper and slings provide plenty of the proper copper colored coil stops to use in flipper rebuilds, and years ago I bought a ton of some random coil plunger from gottlieb via marco for $2 each that were the proper length (although they aren't the stepped kind....)

You can swap your sling coil stops to your flippers and usually be ok, the solenoid pulse for the slings on stars is so short I doubt you'd bottom out on the (presumably) hammered coil stops you have in the flippers now.

Parts availability, mainly. As a disadvantage, I don't think you get get the proper flipper bat with the metal plate running the whole length of the flipper though, so your flipper will feel/perform differently. Not that you can't adjust to its feel of course, some people like that the moving mass of the flipper is lessened. I have a mix of parts in mine.

Oh, the other disadvantage of the pinball life parts is the flipper link while similar in look, isn't made of the same near indestructible plastic that the original stern type 2 links are made of. Pinball resource may still have the original ones available but last time I ordered them was Jan 2017 so they might be sold out of them by now.

The biggest issue with used parts is when the plate is so hammered that you can't get the coil stop mounted correctly (lots of drilling bigger over the years....).

The gen 1 plates can take nuts on the backside if needed, but it makes it a PITA to take the flipper apart since you have to remove the bat for anything.