Keep in mind a couple things:
-Wheel Spacers are often the cheap poorly machined/drilled glorified washers most people use on their front wheel drive rice rockets to fit improperly matched wheels to their car.
-SpiderTrax and similar parts are more accurately "Wheel Adapters" (though Googling "Wheel Spacers" brings up a LOT of similar products to SpiderTrax) and unlike wheel spacers physically bolt to the hub like a wheel and have a second set of lug studs at the ends. Essentially you are bolting on an extension to the hub, where as wheel spacers just hang loosely and reduce the amount of lug threads available for the nut to grab (which is why they are unsafe.)
-SpiderTrax is a sponsor of many hardcore off-roading teams who beat their vehicles severely, if their product weren't up to snuff; I doubt professional rock-crawlers would use them on their rigs.
-As long as you follow proper installation instructions and occasional torque checking of the wheel adapters, there should be little to no issues caused by the wheel adapters. While there could be a minor increase of load on the wheel bearings, it would be little to no different from installing a wider/farther offset wheel in comparison of leveraging forces.
Wheel Spacers of various styles:
These just flop over the studs and in most cases are not even bored to match the hub so they don't sit properly and/or need centering rings.
These are SpiderTraxs installed on a Tacoma hub:
Note that they are bolted onto the existing factory studs and are machined to match the hub bore, then there are new studs to retain factory stud length and strength.