IIRC, a gua'ld can be removed from a human host, but only with advanced technologies only held by the Tok'ra and Tollan. Also a human can survive the death of the inhabiting gu'ald, but only if that gu'ald makes a conscious effort to save the human's life which only happened on two occasions (Jolinar and Jonas' girlfriend).
With Trills, it's explicit that the host's immune system becomes completely dependent on the symbiont.
Exactly, in fact a complete reversal of Trills. Jaffa do not blend with the symbiote, but have no immune system, whereas a full host can survive removal following blending with little or no ill effects (unless the symbiote fights the procedure tooth and nail).
Trill symbiotes blend with their host but after a short time completely destroy the host's immune system, so no matter how benign the removal, it's almost certainly fatal to the host, so only done in the most dire circumstances.