The biggest issue that I have found is that HID/LED don't necessarily have the same filament size (replicated) as that of a incandescent/halogen bulb. So what happens is in the reflector style housing, the light disbursement gets all jacked up and throws light all over the place without focusing properly down the road. NOW, there are some brands that have been able to replicate this filament size close enough that it works close to what a halogen bulb does in the reflector housing.
I have been running the Diode Dynamics SL1 for about a year now with zero complaints. Light pattern seems great and are nice and bright without being too bright. They were definitely on the tight side getting the back into the housing but that was more because of the rubber o-ring. After looking at their instructions they did say that their o-ring caused issues in some housing so to swap out with halogen one and BAM worked like a charm.
I have also seen that
@LXPlore and MisterMemo are running the LASFIT replacements (I believe they both are at least) and have had good things to say about them.