Most fixtures come back without a fight. A difficult restoration is the kind I remember, and I've had more than a few that made me earn it.
The hardest cases usually come down to a few things. A fixture that's mostly there but seized solid, sockets that won't come apart, fittings frozen with a century of corrosion, where every step risks shattering the piece you're trying to save. You go slow, you find the way in that doesn't break it, and sometimes you lose one anyway. Those are the fixtures where patience is the whole skill. Then there's the missing-parts problem. A fine piece down an arm, or short its original glass, or missing the one small fitting that holds it together, and no catalog to order the replacement from. Those turn into hunts, sometimes long ones, matching an uncataloged part by knowing the field well enough to recognize its twin when it surfaces. I've held pieces a long time waiting for the part that would complete them. And there's the material trap, the fixture that looks saveable and isn't. Part of the hard-case skill is knowing when to keep fighting and when a piece has told you it's done. The reason I take the hard ones anyway is usually that they're worth it, rare, or fine, or saveable when nobody else would bother. A fixture that fought its way back carries something an easy one doesn't: proof it was worth the trouble.