They aren't a terrible solution. The flasher is certainly easier, although aftermarket flashers can be finicky. I've seen some say the EP26 and LM470 flashers have some inconsistent blink rates. Not sure why that is but I don't ever see that complaint about the factory flasher. So I can see some merit to keeping the factory flasher and wiring in resistors to take the place of bulbs.
Modern vehicles all have computer controlled flash rates that are based on monitoring amp draw and they will speed up or keep the blink normally depending on the load realized. So on a modern vehicle if you are replacing glass bulbs with LEDs, you HAVE to use resistors, or use crappy bulbs that have resistors built in (usually fail after a while due to all the heat killing the bulb).
I'd say the success level is directly proportionate to the competency of the installer and the connectors they use. If you use good splices with a tool that crimps them tight and follow up with good weatherproof heat shrink, you're highly unlikely to induce failure, not more than anything stock anyways. If you go in there with scotchlok or t-tap connectors and wrap them with some crappy electrical tape to seal them, then yeah don't expect that to last.
As for install time, it depends on how easy the wiring is to get to but I can install resistors pretty quick. I also try to install them on anything that isn't permanent if possible. For example if I were changing tail lights and wanted to do resistors, I would wire the resistors to the tail light harness, not the Jeep harness leading up to the lights. That way the resistors are localized to the tail lights themselves. If you ever have to replace a tail light then sure that adds some work but not much and keeps all the custom hackery on the part, not the vehicle.
