Good question Lesely and I will try and answer it for you. Generally, a mature tank has more stable water parameters and also gives the reefkeeper time to learn their tank. You have BTA's which are the easier of the clown hosting anemones even though they are still classified as difficult. BTA's can be pretty resiliant in a lot of different conditions.
Magnifica (ritteri), Sabaes (crispa, malu), Carpet (gigantea, haddoni) are a few of the even more demanding species. Each anemone has their own set of special needs and setting up your tank to support those needs is crucial in having success. These anemones do not ship as well, may have infections, may need to be treated or may arrive bleached.
Knowing how to handle adverse situations is important. Picking out a healthy speciment is also a big factor. A new reefer may not know what a healthy sabae anemone is supposed to look like and pick out a pretty white translucent one that is bleached and very sick. When you get to the more demanding species named above strong lighting, flow and paramters have to be dialed in even more. For example, a gigantea loves a lot of flow where a haddoni doesn't want near as much. It can be very difficult to tell the two apart since they are so similar. The malu is very delicate and wants to be in a deep sandbed so a 2 inch sandbed may not be enough to keep it happy. The ability to give the anemone what it needs so it doesn't wander about the tank stinging and killing corals is also another reason they can be difficult. The size some of these anemones can achieve in captivity is another reason some of them are more difficult.
Some anemones don't get along well together and chemical warfare can/will be used thus crippling the weaker anemone or both. There are many other factors that make anemones difficult to keep and I have just touched on some of my top reasons. I am glad you are having success with yours but remember not everyone have that same success. Researching your specimen, setting up your tank to support that specimen and picking out a healthy specimen are huge keys to success in the home aquarium.
BTA's are beautiful but remember their sting isn't nearly as powerful as the others mentioned here. They can adapt better then the others I mentioned but that still doesn't make them easy. Just keep doing what you are doing because it appears you have some beautiful specimens:thumbsup: