Building slips with a soldered construction is much easier than one with plastic chairs as some of the rails are short and have other rails close by either side, which makes holding them in place with just plastic chairs difficult.
The difficult bits have been mentioned already and it is interesting that some people hinge the blades and some don't. I have found that the critical factor is the length. If you are building a 1 in 7, there are enough sleepers to enable a blade to move without hinges but with a 1 in 6 or shorter, you get into trouble as you only have one or three sleepers to provide the fixing.
Getting 2 blades to meet up with the stock rails exactly together on a double slip is key to success, as is the alignment through the centre of the slip.
Having said that, with care and not rushing, I built a double slip and a three way tandem very early in my point building to see if I could and both went well enough so they are not beyond a relative novice with the right tools, techniques and attitude!
Certainly agree with you about building slips with ordinary chairs can set some challenges, but lets face it using the incorrect parts building anything will possibly cause problems
Exactoscale do a range of 4 mm scale chairs for situations where non standard chairs are required, in fact by using the appropriate chair it is quite possible to build turnouts and crossings (certainly to the 00 gauge variants and EM gauge) with the minimum amount of soldering and in fact without having to get a soldering iron into very confined spaces. It may also be possible to use this method in P4 gauge but I have not tried it yet
The other critical measurement is that between the tips of both Vees, they must be at the exact distance apart to avoid kinks in the crossing
These chairs are for either the centre of diamond crossings (2 packs required) or the non slip road on a single slip
These chairs are for slips, one pack required for single slips, two for double slips.
There is a quick dodge for using the Exactoscale 0.8 mm check rail chairs both for the 00 gauge variants and EM gauge turnouts and crossings, and I have used these to set wing rails in place on common crossings
Various gauges have to be used at all times, but using plastic chairs on plastic timbers and sleepers is far easier than soldering rail to copperclad strip. If using ply timbers it is wise to make both common and obtuse crossings as sub assemblies using either wafer thin copperclad strip or shim left over from brass kits.