Welcome Visitor! Please take a few seconds and I'm with Shran, no way in h3ll would I run washers.
I added Eibach 30mm hub centric spacers to my daughter's '98 Ranger to run Mustang Cobra rims (Stock rim's offset was 0mm, Mustang's was 30mm) back in '15.
Spacers were carefully installed to ensure they seated on hubs: some fine sanding off of rust was required, then Locktite blue applied and they were torqued down. Mustang rims were installed and they were also correctly torqued. Note: 17" Cobra's in summer/16" standard in winter. Spacers are checked with torque wrench when wheels have been swapped but have never been found loose.
My daughter has been known to drive the truck like she stole it. She also moved from Edmonton to Regina, back to Calgary and then back to Edmonton with truck loaded to capacity/towing a 4x6 u-haul.
So, truck has run for ~3.5 years/60k km with no issues.
From what I have seen, horror stories are one or more of following:
a. Cheap knockoffs where they haven't been hub centric/substandard material /incorrect bolts. Spacers need to be heat treated aluminium, e.g. 6061 T-6 or quality steel; "nail head" bolts need to be used - not ones for steel axles which will pull right through aluminium.
b. Incorrect installation - not seated against hub, not correctly torqued (use a torque wrench; "tight enough" doesn't cut it), wheel not seated on spacer e.g. studs needed to be shorted or replaced with shorter one.
c. Asking too much of the spacer/adapter. Running 4" aluminium spacers on back of dually to fit wider tires and then hauling >GVWR/GCWR, hitting curb hard enough to flat tire, etc.
