I have a 16 foot fiberglass A frame ladder which weighs about a 100 lbs. I saw a ladder leveler on Amazon and have read comments on it. From the comments, it appears that one need to drill holes into the legs to install the extender. Amazon shows the leveler on an aluminum ladder. Thought I ask you folks if you had one if these leveled installed on a fiberglass ladder or if it be okay to drill into the fiberglass without compromising the structural integrity.
I expect that the compromise to structural integrity of the ladder would depend on where the hole is drilled and how large is the hole. Generally, I would not be overly concerned about holes drilled through the web of the leg. After all, there are likely already holes here, such as for the rungs, or for braces, or other items. This area tends to be a relatively low stress area.
Personally, like Snav, I prefer levelers that require no permanent modification to the ladder. Mine are this way.