What are the disadvantages of shadowing?

The main disadvantages of shadowing: it's awkward for absolute beginners, it builds no vocabulary or grammar on its own, it needs clean native audio in the right dialect, and done on autopilot it turns into mechanical repetition with no comprehension. It's a sharpening tool, not a full method.

• Rough for beginners. If you understand almost nothing, you're just parroting sounds, which is tiring and doesn't stick. Shadowing pays off once you can half-follow the clip.
• No new words or grammar. You can shadow a line flawlessly and still not know what it means, so it can't be your only study.
• Garbage in, garbage out. Shadow a robotic voice or the wrong dialect and you drill the wrong rhythm. If your target is European Portuguese, shadowing a Brazilian clip teaches the wrong accent.
• It can go mindless. Echoing on autopilot with your brain switched off builds a fluent-sounding parrot, not understanding.

None of these is a reason to skip it, just to use it right: keep clips short, make sure you actually understand them, and run it alongside real input rather than instead of it. It helps to know first what shadowing actually helps with, so you aim it at the right job.