I only have a single experience with a UG splice, but it only held up for 2-3 years. This was a heated waterer in a pasture that my wife wanted moved (most of her reasons were good, but it did make for some work.) Spliced into the existing feed to the waterer. Maybe 2 years later have a dead short somewhere in the line. Can't say for certain it was the splice, but seems likely to me. Abandoned that branch circuit and installed new.
I like the idea of setting a pole light and haveing a box to make the splice, but that might not fit into your plans. As others have noted, you don't have an abundance of capacity in the existing wire, but if you are only looking for a few lights and a convenience receptacle or 2 it should be fine.
Good Luck whatever you decide,