Bulbs are tricky because there are two different zigbee profiles used for them: zigbee home automation profile (ZHA) which is the one used by smartthings, and the zigbee light link profile (ZLL) which is the one used by the Phillips hue bridge. And this can get confusing across regions: for example, LIGHTIFY bulbs are ZLL in Europe, but ZHA in the US.
http://thingsthataresmart.wiki/index.php?title=Repeaters
issues with individual brands
The US Osram lightify bulbs when connected directly to the SmartThings hub and on power will reportedly act as repeaters for other ZHA devices on that smartthings network, But there have been a few reports from community members who said it didn’t work while others said it did.
Hue bulbs connected directly to the SmartThings hub without a Hue bridge definitely will act as ZHA repeaters (the bulbs shift to the ZHA profile when connected in this way), and a number of members are using them–but they have proven to be less reliable than other device classes. The $15 Hue white is a very inexpensive zigbee repeater, but it may only be reliable for other bulbs. And Remember that the bulb must be on power in order to repeat. It doesn’t have to look like it’s on, but it does have to be receiving current.
Also, officially SmartThings recommends only using bridge-connected Hue bulbs because the Hue bulbs cannot be reset on their own. So once you connect them directly to the SmartThings hub, you cannot move them back again to a Hue bridge unless you buy an additional device to do the reset (usually the Lutron connected bulb remote, $30 at Home Depot. You reset one bulb at a time, so you only need one of the Lutron devices even if you want to reset multiple bulbs. )
Cree bulbs can be used as ZHA repeaters, but they appear to be missing a couple of Zigbee clusters, so they aren’t always reliable repeaters for other ZHA devices. Some people use them and they work great, some find that some messages get lost.
GE link bulbs are unreliable. They tend to drop off the network every so often. This is a firmware problem that GE has acknowledged, and is the reason why these bulbs are not on the official SmartThings compatibility list. People still use them as lights because they are very inexpensive, although the late 2015 price reduction on the Hue whites puts them in the same range. But the GE links really cannot be recommended for use as repeaters because their unreliability puts the other devices depending on them to repeat at risk.
So with bulbs there isn’t quite the same standard as there is for other mains powered devices, and there are two different bulb protocols, and of course the bulb must be on power if you want it to repeat, so really for those you have to check by each model. Some may work, but some will not, and some seem to be unreliable.
when you add a new repeater to your network
Finally, and this applies to any repeater, not just bulbs, when you add a new repeater the other devices on the network will not be aware of it until you do a zigbee heal. It’s easy to do: remove the batteries from the hub and take it off power for at least 15 minutes. Leave all the other devices on power, including bulbs. Then put the hub back on power. At that point, all the devices on the network should rebuild their own neighbor tables. This can take a while to complete, so you might not see improvements until the next day. But it should ensure that any device that is within range of a bulb that can repeat for it will use it in the future.
Sorry, I know that’s a long answer, but bulbs are complicated.