You will always find good and bad reviews on google for all builders. If you want to find good reviews - you'll find them; if you want to find bad reviews - you'll also find those.
William Lyon as other builders all have inspections done to ensure they're being built to code. You get to pick and design the inside of the home - so you are dictating how it will look. The materials aren't usually anything spectacular or particularly special; it's just custom to how you like it.
Personally, I like William Lyon homes and have not heard anything bad about them but I am sure they're not perfect.
I myself have owned William Lyon homes and was very happy with them. I have not in all the years I have been in the RE Field heard anything derogatory about his homes.
Check out the home you choosecarefully for the quality of the construction and all the upgrades.
If you are buying a new home the builder usually has a list of the brands they use for their construction. Ask what brands of windows, sinks, fixtures ect. that they use. Then check that out for yourself.
Best of luck to you,
Prudential CA Realty
OC Homes Realty