  We discuss the properties of bound states in finite-bandwidth waveguide QED
beyond the Rotating Wave Approximation or excitation number conserving
light-matter coupling models. Therefore, we extend the \emph{standard}
calculations to a broader range of light-matter strengths, in particular, in
the so-called ultrastrong coupling regime. We do this using the Polaron
technique. Our main results are as follows. We compute the spontaneous emission
rate, which is renormalized as compared to the Fermi Golden Rule formula. We
generalise the existence criteria for bound states, their properties and their
role in the qubits thermalization. We discuss effective spin-spin interactions
through both vacuum fluctuations and bound states. Finally, we sketch a perfect
state-transfer protocol among distant emitters.
