Pre-wired fishplates...

[swisstrains]

How about a compromise solution - soldering droppers to normal fishplates, on the bench, where access is easy and there are no plastic sleepers to melt?

This is what I did when building my layout.
Soldering wires onto the bottom of fishplates makes for a neat job but the electrical connection is only as good as the fishplates mechanical connection and can deteriorate over time. In 15 years I have had two failures where I have had to "jiggle" the offending fishplate to restore connectivity. Ideally separate feeds need to be soldered directly to each section of track.

[ntpntpntp]

Yeah, soldering directly to the bottom of the rail is the best solution, I'm not a fan of pre-wired fishplates as you're still relying on a mechanical connection as @swisstrains says. However I do appreciate that for those not having the dexterity to do the soldering themselves then the pre-wired fishplates are the probably the next best solution.

Fortunately over the past 24 years of exhibiting the layout I've only had a couple of cases of conductivity problems due to fishplates, and they were either side of a bridge that I'd set into the trackwork after the rest had been laid, and after some scenic work had been carried out. I gave the fishplates a good squeeze and ran a little solder in (I don't usually solder fishplates at all) and they've been fine ever since.