Triacs have a poorer dv/dt and di/dt rating, so they are susceptible to spikes. Because they are bidirectional, they are tough to get working with highly inductive load or load that has non unity power factor. Always better to use back to back SCRs, though Thomson CSF did make more rugged triacs, I do not know if they are available.
Back to back SCRs require slightly more complex firing circuit as compared to Triacs, so for simpler apps, triacs make the whole circuit cheaper.
But quite a few device manufacturers do make back to back SCR modules for high power apps.