Pete I have no experience of 6v LEDs. But I did for a short while play around with 12v ones. Two problems to my mind.
Yes very bright immediately behind (or at the front) of vehicle. But poor coverage obliquely or at the side, much poorer than a filament bulb. Some replacement units have a few sidewards facing LEDs but even these have a limited arc of coverage.
If your using them for turn indicators bear in mind the flasher is likely to be a thermally switched. To operate it needs the load of the filament to draw a current to make it flash. The slight current drawn by the LEDs is insufficient for the flasher to work unless you change to a solid state flasher. The other alternative is to wire a high wattage resistor across each LED turn light to draw significant current to make the flasher work.
On other forums an issue has been raised that in the event of an accident having non original lighting may not be compatible with insurance cover. Whether that is a real issue I don't know as my LED conversion did not progress as I felt the light visibility obliquely was reduced & hassle with changing the flashing system.