The companies they own are pretty diverse (capital markets, liquidations, retail/brands, internet isp, magicjack). So I dont think they're really constrained in where they can reinvest capital. That's just a function of there being good deals out there.
They've done great and reinvested, but I don't think it's really a rapid growth company. They've steadily reinvested cash, but in 2020 equity capital markets volume exploded immediately after liquidations exploded. So I don't really see those as true growth, as both are episodic in nature. They're good at seizing those opportunities though, as valudontlie said.
No idea on valuation. It just depends how long equity capital markets volume is strong. It could collapse in 2 weeks, it could go another year. If it collapses in 2 weeks, RILY is probably overvalued. If ECM goes another year, RILY is an easy double.