In Skyblock, players begin on a tiny floating island and build up from a few blocks. The mode lives or dies on its progression: what players must do next, what they can buy and how islands grow. A Skyblock server setup packages the parts of that loop, an island manager, a way to expand and upgrade islands, challenges or missions that guide new players, and a shop or economy for trade.
Islands are the heaviest part of the server, because each one is a protected area with its own members, settings and limits, so the way a setup stores and loads them affects performance as the player count grows. Many setups also add generators, minions or custom mobs; each is an extra system to maintain.
What to look for in a Skyblock setup
Island plugin: find out which island plugin it is built on and whether that plugin is still updated for your Minecraft version.
Progression: look at the challenge list and upgrade tiers, and check that you can edit them without rewriting the whole configuration.
Economy: see how money enters and leaves the game, and whether prices are easy to tune so one source does not break the economy.
Performance and world setup: check recommended player counts, world generation notes and how islands are unloaded when nobody is on them.
Updates and licence: confirm what updates are included and whether you may run it on more than one server.