Let Γ be a countable discrete group, and let π:Γ→GL(H) be a representation of Γ by invertible operators on a separable Hilbert space H. We show that the semidirect product group G=H⋊πΓ is SIN (G admits a two-sided invariant metric compatible with its topology) and unitarily representable (G embeds into the unitary group (ℓ2(ℕ))), if and only if π is uniformly bounded, and that π is unitarizable if and only if G is of finite type: that is, G embeds into the unitary group of a II1-factor. Consequently, we show that a unitarily representable Polish SIN groups need not be of finite type, answering a question of Sorin Popa. The key point in our argument is an equivariant version of the Maurey--Nikishin factorization theorem for continuous maps from a Hilbert space to the space L0(X,m) of all measurable maps on a probability space.