Mechanically, there's nothing more difficult about rebuilding a mag than there is about, say, assembling a valve train with all the proper clearances. If you're comfortable with electricity, it's no more difficult than, say, repairing a toaster oven whose heating element has become disconnected. However, it does take a few special tools (all of which can be purchased or fabricated), and the right condenser.
The condenser has been the downfall of many a rebuild over the years, by professionals as well as amateurs, because much, much misinformation exists about what properties they need to have in order for them to survive the high current pulses generated by a magneto. A physicist conducted a years'-long study of possible replacement condensers, subjecting them to a rigorous set of tests, and publishing the results in 2011 in a two-part series in the Antique Motorcycle Club of America's magazine. Unfortunately, the Panasonic condensers he recommends are no longer produced, and the stock of them sold out as soon as the magazine appeared. Luckily, I have a lifetime supply, so I'm all set, but currently this is a problem for others. I won't say anything bad about any of the various condensers touted by one person or another, but only repeat that much, much misinformation exists.
Another problem is magnetizing the magneto. Removing the armature from a good magneto causes its alnico magnet to immediately lose ~15% of its strength, resulting in the energy in the spark dropping by over 25% (for pre-WWII bikes that use magnets other than alnico, the demagetizing field is much greater, and they lose quite a bit more). Not everyone who rebuilds magnetos has the very large electromagnet that is needed to restore the field once the rebuilt armature is returned to the body, but since bikes will still run with a 25% reduction in spark energy (although they are harder to start, and miss when the mixtures are too far off), many magnetos are returned to the customers operating only 75% as well as they could. The field strength required to remagnetize a magneto is described in a Lucas technical bulletin. Several years ago I fabricated my own electromagnet that I designed to give 20% higher field than called for, just to be sure. I have no way to weigh it, but it's easily well over 200 lbs.
Yet another problem is the coil. Dave Comeau posted a comment that he has found most original coils to be good, and don't need to be rewound. I agree. I do have the appropriate testers to ensure the coils are good before reusing them. But, if a coil is not good, a replacement needs to be located. This is the one place where I still have to rely on outsiders, since I've yet to break down and start winding my own replacements. I could do it if forced to, but its such a fiddly job that instead I've adopted the 'trust, but verify' approach. The few times it has been needed, I've had someone else rewind the coils, but I then subjected them to extensive high voltage pulse tests at somewhat elevated temperatures to make sure the insulation they've used between the layers of windings won't break down in service.
Although life would be simpler if one could simply pay money to a self-described magneto rebuilder and have a properly refurbished magneto returned to them, life doesn't always work that way. So, either you have to find a rebuilder who really does know what they're doing, or do it yourself. Unfortunately, glowing recommendations are to be found for people who I know are not properly equipped to do a proper job (other than making the outside of the mag look pretty), as well as for people who are equipped. So, separating the good rebuilders from the bad is a quest in itself.
I'll end this by saying I have no personal experience whatever with magneto restorers who post on BritBike Forum, so nothing I've written should be taken one way or other as being about them. I restore my own magnetos myself, and am happy to offer advice to anyone interested in magnetos, but my lack of personal experience with magneto restorers means I am not in a position to make recommendations about any of them.