The biggest key factor is the lack of G3 or super G3 support for the ATA186. It is not very scalable and can only support rates upto 14.4K whereas Super G3 uses 33.6K to connect.

One other possible concern is whether you require either Cisco or the ITU T.38 fax-relay support, as neither are supported.

Only fax-passthrough is supported, so depending on where your originating gateway is and they are routed across the WAN, you may have to reconfigured your transit voice-gateway for pass-through?

Configuring FXS interfaces will require more administration/configuration depending on whether your gateway is H323 or if you require SRST.

ATAs may provide less administration and simpler to deploy in comparison. It really depends on your requirements. My preference would be for FXS but ATAs may suite your deployment?

Rgds

Allan.