It can be done very safely. In fact Dave Yates told me that the steerer on my forks was stronger after the weld than before as he sleeves, pins and brazes the steerer.
Yates, Argos, Mercian, Jackson and every other reputable frame builder offers the service - they are not likely to risk reputation by doing something suspect now are they. It's not a new service but something that has been done for years.
As for steerer extenders: a quick way to add half a pound to your bike, limit your bike to aheadsets only and make it look like a bodge job.