Limits

The server has limits on certain things to prevent lagg and server crashes.

Island Limits:

Skyblock has limits per island for certain blocks. Here are a view examples. use /is limits for an in game gui with all the limits per island and how many blocks you have on your island. 

Hopper: 10000
Piston: 20000
Sticky Piston: 8000
Cactus: 1000
Dispenser: 10000
Dropper: 10000

Redstone limits:

Fast redstone clocks are stopped by the server. Chunks with too much redstone activity will be unable to use redstone signals for a couple of minutes.
The limit is subject to change depending on player feedback and server performance.

Redstone block limits:

There are limits per chunk of how many of a certain block can be placed per chunk.

Pistons: 1024
Sticky Pistons: 384
Hopper: 512
Dropper: 512
Dispenser: 512
Observer: 64

Entity limits:

There are limits for mobs per chunk to prevent server and client lagg or crashes. If the limit has been reached mobs won't breed or spawn and if the limit has been exceeded mobs will be removed.

There is a limit of 25 for friendly mobs / animals per chunk.
Large animals have a limit of 10 per chunk.
Animals/friendly mobs with a nametag will not be removed.
Tamed mobs will not be removed

There is a limit of 16 for hostile mobs per chunk.