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
Custom RPC Return Types
RPC Overloads
Incremental Update Loop
Custom Messaging System (Unnamed)
Custom Messages (Named)
Per Client RPC
Bulk Client RPC
Connection Approval 1
Message Encryption
Message Authentication
TLS Certificates
Custom Tickrate
Synced / Networked Vars
Custom Sync Structures
Unlimited Networked Vars
Duplex Networked Vars
NetworkedVar SyncToOwner 2
NetworkedVar Custom Write Targets
NetworkedVar Custom Read Targets
Multi-project Support
Built-in Lag Compensation
BitWriter/BitReader (or similar packer)
Network Profiler 3
Pooled Memory Management (Internal)
Area Of Interest
1 The library does not support message authentication or message encryption. By default, authenticated messages are insecure.
2 This feature is limited to individual MonoBehaviours. You cannot change the syncronization of a single field.
3 This feature is not included in the library and has to be purchased.