The MLAPI is a library with strong development behind it. As a result of that, it has many powerful features that competing libraries don't have. Here is a comparison of features between competing libraries.
If anything is incorrect or missing from the table. Please submit a pullrequest with a correction.

Feature MLAPI Mirror HLAPI
Host Support
Spawn Management
Messaging System
Custom Transport
Multiplex Transport
Scene Management
Behaviour Based Messaging
Network Time
Send Channels
Custom RPC Parameter Types
RPC Return Values New
Custom RPC Return Types New
RPC Overloads New
Incremental Update Loop New
Custom Messaging System
Custom Messages
Per Client RPC
Bulk Client RPC
Connection Approval
KeyExchange
Message Encryption
Message Authentication
TLS Certificates
Custom Tickrate
Synced / Networked Vars
Custom Sync Structures
SyncedList
SyncedSet
SyncedDictionary
SyncedEvent
Unlimited Networked Vars
Duplex Networked Vars
NetworkedVar SyncToOwner
NetworkedVar Custom Write Targets
NetworkedVar Custom Read Targets
Multi-project Support
Built-in Lag Compensation
BitStream
BitWriter/BitReader (or similar packer)
Network Profiler
Relay
Pooled Memory Management (Internal)
P2P
Area Of Interest