is securing a 2x4 onto fibreglass with liquid nails strong

Depends on what you mean by strong.

If you want the strongest strength use 100 % clear silicone for that attachment for the smooth surface of fiberglass.

Fiberglass expands and flexes. Liquid nail offers adhesives for that but its not 100% silicone in proprieties.

Which Liquid Nails did you plan to use, there must be about 40 roducts in their line. Fiberglass what, door boat, Corvette... We could do a better job is you actually tell us what you are doing.