Arjay and Nidacore (Nidabond pourable) are both ceramic polyester compounds, as opposed to Seacast which used fiberglass. Both are about half the cost of Seacast which makes them very attractive. Nidabond has been used by Wellcraft and Venture Marine for some time now. I think they area viable option if they are done correctly, and the key word there is "correctly".
From their websites:
Description: ARJAY formulates its Ceramic Pourable Compound with premium polyester resins and high strength ceramic spheres resulting in high tensile and flexural strength. This lightweight compound is ideal for filling large volumes where strength and rigidity are major concerns. The Ceramic Pourable Compound mixes and pours easily from the 5-gallon pail.
Features:
Excellent rigidity and strength
Formulated with premium resins
Low exotherm
Mixes easily
Pumpable or pourable
Excellent stability
Uses:
Filling large volumes
General purpose filler
Radius compound
Structural applications where high compressive strength is required
and:
NIDA-BOND POURABLE TRANSOM COMPOUND is ceramic filled polyester exhibiting exceptional physical properties. In particular, the compressive strength has been documented by an independent testing laboratory to be 3,895 psi (ASTM 695). This is several times that of plywood and 8 to 10 times that of PVC foam. In addition, the failure mode shows elastic yielding before failure. This makes the NIDA-BOND POURABLE TRANSOM COMPOUND material an excellent choice for critical applications such as coring transoms on power boat hulls.