While my colleagues have predominantly talked about liquid silicone injections causing granulomas, your question (to me) still seems mostly unanswered. Dr. Aldea has given an answer that outlines why silicone injected in the form of liquid droplets within your tissues is different from a solid implant contained within its capsule (scar covering). He explains why the tiny scars your body forms around silicone droplets (injected silicone liquid causing granulomas) can be extremely difficult if not impossible to remove surgically. Why would you want these silicone droplets out? Because the granulomas may become firm, tender, and can migrate, since there is no solid form to keep them together.
Silicone breast implants, however, are now cohesive, meaning they are a soft solid silicone elastomer that cannot leak or migrate. Truthfully, microscopic molecules of silicone ARE within your body already, since immunizations, tetanus boosters, penicillin or other medication injections, insulin shots, and other injectables (including IV catheters) are all lubricated with liquid silicone, and we all have tiny amounts of this silicone within our bodies.
Silicone is polydimethylsiloxane (PDMS), which is a polymer. The length and cross-linking of the polymer chain(s) are responsible for the physical properties of these substances. Shorter chain lengths of PDMS are oils or liquids, whereas longer-chain, cross-linked, high molecular weight silicones (PDMS) are rubbery elastomers. Even longer-chain silicones can form harder and even less pliable elastomers, like finger joints, pacemaker insulation, testicular implants, and yes, bathtub seal!
The present generation of breast implants are made up of a cohesive, high molecular weight, cross-linked silicone gel that is a soft, rubbery solid all the way through, meaning it cannot leak or rupture. (BTW, this makes the FDA recommendation for periodic MRI scans to look for "silent rupture" in the present generation of silicone implants nonsensical, though this may be appropriate if you have a problem with older silicone implants, which had a elastomer shell containing less cross-linked, spongy, sticky PDMS gels or liquids.)
So, even though injectable polydimethylsiloxane liquid has the same molecules as polydimethylsiloxane solid, the latter does not migrate in soft tissues, does not cause a granulomatous reaction, and is contained in an anatomically usable shape. The liquid silicone injections are "safe" in the sense of not causing autoimmune disease or cancer; it's just that injections can cause these unpredictable micro or macro scars (granulomas) that can be nearly impossible to remove if they become tender, inflamed, infected, or just look bad if they migrate where they are not intended.
To be sure, silicone gel implants can also become infected (requiring removal, but that is easily possible, and thankfully very rare), develop capsular contracture (usually from bleeding or bacterial contamination), or be malpositioned. Artificial hips, pacemakers, or heart valves can also develop some of these issues. How and where they are put into your body is much more of an issue than their molecular makeup! And remember, the present generation of silicone breast implants are a cohesive solid and CANNOT leak--no MRI scan needed nor advisable.
Sorry to be so technical, but I hope that explains things in a way others were not conveying! Best wishes!