The technology definitely has its flaws. It's not currently biodegradable, so you may be stuck with this school of fish unless there's a way to flush them out. Gold and silver aren't the cheapest metals, either. Scientists are working on biodegradability, however, and they're hopeful that it will be useful for more than just guiding drugs. You could use to control individual cells, for example, or conduct certain forms of non-invasive surgery. It may just be a matter of refining the technique before you can get medicine exactly where you need it.