If they have an imprint saying HT, PCP-HT, FCC-HT or KD they are fine to use.
If they have an imprint saing MB they have been treated with Methyl Bromide and are therefore poisonous and should not be used for anything.
My number one use is as raised lasagna beds.
I used to split them in half and build one 4x4 ish bed out of two pallets and 8 deck screws..
Now I use 4 pallets and cut the deck boards off of half of each one.
This leaves 2 feet of 2x4 sticking up, useful for holding trellising or plastic or frost blankets.
I use the removed boards elsewhere.
I just built some nice lumber racks out of pallets.
Most of the lumber on the racks, also from pallets.
I try to use them as is, with as few cuts as possible.
I've been building little cold frames from pallet wood and refrigerator shelf glass.
Rather than buckets or half barrels , going forward I hope to use stacking pallet wood frames or crates to root hardwood cuttings.
My soil sifters are made from pallet wood and various sizes of hardware cloth, as is my potting table.
The main limiting factor of pallet wood is the length.
I use screw laminated posts quite often, but haven't graduated to beams or using glue.
For 6 foot tall fencing, I use two rows of the roughly 40" pallet board.
The top row overlaps the bottom row,rather than them butting together .
This means no measuring or cutting of the boards, and using a brad nailer keeps the fastening cheap and fast.
I have used brads and staples to nail together wooden walkways, just pallet wood directly over dirt and the last quite a long time.