Kentic,

Have used them before in system to make it easy to replace parts. No one can guarantee that they will last years but mine are still good after 5 years. You can check how they are doing if the unions are visible. Your ASV will need attention before the unions. GL sounds like a nice system and your thinking ahead.

At my first house I built my irrigation manifold using PVC unions. I hate things that aren't built to be maintained.

Now I would just buy one of orbits manifolds with the unions built in. They are a less expensive and more compact than big PVC unions.

The end of the manifold is also threaded so you can easily add more stations in the future.

hearing your discription of your planned layout and having built manifolds both way's, unions would save you a lot of trouble down the road.