A taper rebar coupler is a mechanical connector used to join reinforcing steel bars (rebar) in concrete structures, particularly when space is limited for lap splices or when larger diameter bars are used. It utilizes a tapered thread design for ease of alignment and a positive locking connection. 

Despite their numerous advantages, taper rebar couplers also come with some disadvantages:

1. Reduction of Ductility: The process of machining the ends of the rebar into a taper can weaken the cross-section, which may lead to reduced ductility. This is a crucial factor to consider, especially in seismic regions where the ductility of the material is essential for absorbing energy during events such as earthquakes.

2. Alignment Requirements: There is a high demand for precision during the alignment of the rebar ends. Poor alignment can compromise the integrity of the connection, leading to structural failures.

3. Cost Considerations: The overall cost of taper threading equipment and the couplers themselves can be relatively high compared to traditional connection methods. This may pose a budgetary challenge for some projects.