I think it really depends on your space & how much of the work you are willing to do. When I first started doing the research, all I read was that you could never have enough unions & that "experienced" reefers always used them. After approx 4 years in the hobby (still relatively new), I think there's a time & a place.
We have a 300gal DT, & dedicated space in the wall behind the tank so there is plenty of room. I also enjoy cutting & gluing PVC, so that's a factor.
For this build, I did not bother with unions on any of the fixed plumbing (drains, returns or surge device). All of the pipes are large enough they could be snaked if required. Unions are also 10x the price of couplings, so if a piece of pipe would ever need to be cut, it can be repaired easily enough for < $1.
Here's the return chamber (dual return pumps) & the pump for our surge device. This is very much a DIY project (clearly), but it has also evolved from a single 75gal sump into a 3 tank (50gal rubbermaid stocktank refugium, 75gal sump & 18gal surge) biological filtration system of it's own.
The only union you can hardly see on the right is for the gravity top-off that connects a 20gal Brute for a Kalk drip to 1/2" pvc. Calcium will build up in that pipe & an occassionaly dip in vinegar is required. Sediment will also collect in the bottom of the Brute, so having this piece easily removed is important. Otherwise, all glued solid...no leaks.
Happy to take a few pics of the return plumbing that comes through the wall & over the top of the display & the gate valves under the display that restrict the drains if that would help. But as stated, no unions there either.
