Can they be used to glue up pressure treated lumber?

for?

Putting a new roof on the Dog's Dog House. The old wood shingles blew off in that windstorm that knocked out the power for over 1200 customers for 3 days in my Town.

Yes. What you use for spline or biscuits along wth the proper glue type would be the real question.

Osky

Use Gorilla glue or similar glue.....likes moisture. Plate or biscuit jointing works on most wood and is pretty universal.

I use a plate joiner often, they are great tools. I would think they'd work just fine on treated wood if the glue works. If the wood is wet and appearance isn't much of a concern, polyurethane will work great. It expands and foams as it cures and if you use a little too much it can make a mess.
If the wood's dried, there are lots of waterproof glues that should work.

I like a plate jointer but I really like my kreg pocket hole jig. Locks joints really, really tight.

I should sell mine, can't recall when was the last time I used it.

Kreg joints have their place. I wouldn't want to give either one of the tools up. Edge to edge and mitered ends- plate joiner,

I've made a lot of coyote and cat boards out of Western cedar fence pickets that are done with biscuits and held with Elmer's wood glue. Works great if there's no basswood where you live.

I use a biscuit joiner more for alignment than I do for "holding strength". Most projects, I rely on the glue to provide the bond.

If I need more support, then it's either to dowels (if need to be hidden) or to pocket hole screws.

Don't underestimate the power of glue. If done properly, and the right glue is used, the joint should be stronger than the wood itself.

If you're gluing treated lumber, you may have trouble getting good glue penetration, and thus poor bonding. You would probably do best to use a mechanical means of joinery for best results