It depends on how it's done. Thickness is the key. Laying a layer of "surfboard cloth" down and slopping some resin around is what tons of folks consider to be fiberglassed. If a guy layed down 1/4 - 1/2" of glass, the pond would survive a nuclear bomb, well maybe
. You gotta remember that fiberglass came into boats in the late 50's and early 60's. Most of those boats built back then are still floating and very sound in their construction. Today they build boats out of glass in the area of 150' -200' weighing hundreds of tons and they can survive gail force storms out in the ocean.
Glass work can be expensive as Kent stated, but if you can find the right guy(s) that knows what they're doing it can be done for the same/similar price as polyurea, and be a structure as well. But as with everything in life there's a ton more people that say they can provide a service and claim to do a good job, but in reality finding someone to do fiberglass right is like finding a needle in a hay stack. Finding someone that knows what materials are available and how to use them is the biggest problem. Most guys just want to do as little as possible so they don't have to spend very much on materials and more $$$$ goes in their pocket, which is where I see failures happening giving fiberglass a bad name. I have heard pond builders talk of going in and "pealing glass out of a old structure, it didn't stick, there was cracks, etc" If you lay enough glass down it won't fail, and the glass becomes the structure if the substrate fails.
Here's a example. In the late 80's is when I got my start on boats. Marine surveyors have to survey boats every so often as required by insurance companies or when a boat is bought or sold. When some boats are made, they have wood fiberglassed into the hull for rigidity and added strength. The wood would rot over time, the surveyors would find the "hollow" sounding spots in the boats hull/structure, and my job was to fix it. I would sometimes gut the boat down to a bare hull on the inside because the rot was so bad.
But, lots of these boats were still being used all the time, and even with the wood rotted away, they still functioned just fine. Granted I wouldn't want to take the boat out into a heavy storm in the ocean, but for going up and down the river they still functioned.
My pond has
No structure around it. We dug the hole, stood 1/2" 'Hardibacker' cement board up for walls, stapled 18"w aluminum roof flashing into the corners to make the radius's as well as to the dirt floor, and layed down 2 layers of very strong/expensive cloth which 2 layers adds up to a thickness of about 1/8". Once I was done glassing and gelcoating, I filled the pond up with water not having any backfill around the walls, and it held water just fine overnight. I wouldn't recommend doing this, but it was my test.
I drained the pond, backfilled the walls with dirt, re-filled, and tossed the fish in. My pond from digging to tossing fish in took 2 weeks with one of the weeks getting rained out.
To me the best thing about glass, is
if it does fail, it is very easily repaired, and can be repaired 30yrs down the road without having to re-do the whole thing.