My idiot contractor hired some dummies to do the flooring install in my house. I'm about 99% sure they forgot to install underlayment, since they left 3/4 of the underlayment material a I bought unopened.

The flooring is a click-lock, stranded, solid bamboo from Floor and Decor, 3.5 inches wide, on a concrete subfloor. The floor feels like it might a little bit wavy, and there is some very light crackling/popping under weight, but is otherwise ok. They did remember to let it acclimate.

What are the possible consequences? Is it worth pulling up and putting underlayment underneath, or should I just let it be?

Pulling the baseboards is not an issue, because they screwed that up too, and I have to do it anyway. They may have also forgotten to leave a proper expansion gap, but I won't know for sure until I pull the trim.

Seen that, especially with attached cushion where people believe that is a vapor retarder. It needs to come up as the moisture will destroy the floor and mold may invade the locking mechanisms.

Apparently the store gave my contractor twice as much material as they should have (800 sq. ft. instead for 400 sq. ft.), which is why there were rolls leftover.

There are some spots where it feels like it's definitely missing though, and there are multiple other issues.

It used to be that 3 inch flooring could be recommended for a floating floor.
Now only 4 inch and wider planks should be used for floating floors. I think that is to support the weight better without deflecting as much.
I would not recommend anything under a 4 inch plank for floating over a slab or any other sub-floor.
Some underlayment will compress in high areas and bottom out when the floor below is not flat and has a bump.
This helps make the floor flat. Did the contractor check the slab with a straight-edge?
Did they take the time to read the instructions that came with it?
Do you have a copy? There is usually a copy in each box, or package.

I don't know what the contractor did and do not have the instructions. I do know he really screwed up a couple other things on the install, and I will put the pictures in another post.

As far as the floor flatness and material quality are concerned, I'm not worried. I personally installed the same materials in another bedroom in the same house last year, with the same underlayment and enough expansion gap. It's had zero issues.

This year I hired this contractor on recommendation from a friend, and have had nothing but issues with quality and his guys taking shortcuts on every job they've done.

You may find the instructions online. They are not a secret.
Having a flat floor is the number one requirement.
Everything goes down hill fast if the floor is not checked before installation.

Thanks for the suggestion. The instructions are not online.

When I personally did the flooring in the first room, I rolled the dice and eyeballed for flatness, and did a quick once-over with a 2x4. Everything was fine. The contractor did go through the effort of grinding off the carpet glue that was in one room.

Also, for context, the rooms are fairly small ~12'x13' and ~13'x14'.

Sorry you still need to re-do the base board. If there was a moisture barrier under the floor, then the foam underlayment is for comfort and to help keep the floor from tapping when it is walked on.
You will need to take some apart to check for the moisture barrier. If it is under the last row, it is probably under the whole floor.