With a normal holy symbol I'd guess probably nothing. I don't think (un)holy symbols are magical in of themselves, just conductive for channeling divine power. Maybe. Although if we're talking about Xykon's phylactery... good question. Disenchanters are high enough cr that it wouldn't be too silly for them to pose a threat to even something like a lich, but I'm not sure if holding a liches soul should/would be considered a "special ability" that can be drained. Or even how phylacteries actually work, which isn't percisely spelled out in the monster manual and I'm unsure if it was ever printed elsewhere. Although it'd make sense I suppose.
The bigger issue the disenchanter would face I notice, now that I'm rereading it's stat block, is when sucking from magic items with charges it sucks 10-40% of the current charges. So if you had a wand with 10 charges it'd suck like 4, then 2, then 2, then 1, then be stuck I guess? Suckling on that last remaining charge like me trying to get the last of the peanut butter out of the jar for a 3AM sandwich?
Every year hundreds of disenchanters starve to death trying to suck out the final charge from a wand. For the low low price of an enchanted shield per month, you can save a life. Call now.