Man, my buddy had the exact same experience last summer. He was doing a fence line in Tucson and the homeowner brought in these composite posts, my friend was ready to argue against them. He said after the first monsoon season, when all the wood posts nearby were already starting to look rough, these things hadn't moved a hair. He won't shut up about them now, says it's the only way he'll do a job if the client will pay for it.

That "thick mat of roots and plastic bags" sounds like a real nightmare. A buddy of mine was doing some dredge work in an old canal and pulled up what they thought was a huge log. Turned out to be a fully intact, rusted-out bicycle frame from like the 1950s, completely tangled in fishing line and crab pots. They spent an entire afternoon just cutting it all free, and the magnet they brought in to help just stuck to the bike. Total mess that cost them the day.

How deep did you set those posts in concrete? My uncle in Joplin had a similar setup and a tornado watch last year made him realize his were only 18 inches down. He had to redo the whole foundation.

An inch too short is not exactly a world ending problem.

Saw a forum post about using a metal mesh shield for that exact bus interference problem.