I've had my biocube 29g for a couple years and still love it, even though I have 5 other tanks going lol.
It is simple way to get into the hobby and the tank isn't so small that you can't have several fish, if chosen correctly. It is also cheaper to go with a self contained system. The only thing you can't really grow under the stock lights are hard coral, SPS
which are the thin branchy ones. The soft corals and flowy stuff will be fine under the lights.
I would recommend upgrading the back filter section from the bio balls to the media basket such as the one from intank. They even have a fuge basket too that you can get if you wanted to add macro algae for nutrient export. I run a bag of chemioure elite, purigen and a filter floss pad and change out the media about every 3-4 months and the pad a couple times a week depending on if its grungy or not. lol
For reference I have the following in my BC29g- an itsy bitsy sailfin blenny, a chalk bass, a pygmy hawkfish, a royal gramma and a black combtooth blenny. As far as coral i have maybe 20 different kinds and they are all doing well.
The only thing I wouldnt recommend is an anemone. I have one in there now, a red bubble tip and he is just not doing well, its the lights and I recently started running a 4 bulb t5 fixture over the cube in place of the stock hood just to see if I could get the anemone to get happy. No luck so far....