Much of what is done in drydock is below the waterline or out of view of the passenger. There are required maintenance and inspections, which require the ship to be out of the water.
Yes, they can do passenger facing work as well...new carpets, beds, other work. An amazing amount can be done in a very short time, because they plan these things to the minute, have all the supplies and equipment ordered and received before the ship even gets out of the water. Much of the crew is kept on board to help and support the work. Of course, they go 24/7 on drydock work as time out of the water is money lost to the cruise line.
I also take reviews with a huge load of salt. A lot of reviews are about food or service...which are very subjective and of course, can't be fixed in a dry dock. Other things like too much (or not enough) A/C are also subjective, and not really flaws that need a fix. Things like plumbing frequently are fixed on board...maybe with in a day or two, and the reviewer focuses on the problem, not the solution. The list of possible complaints goes on and on...but hopefully you can tell apart things that are serious mechanical issues, those that are lesser mechanical issues, those that aren't really mechanical issues at all, and those that are totally subjective.