I looked In to Ali undertrays last summer. The pros are:
They don't fall to bits as easily.
They offer slightly better protection from stones, bumps etc. in the main areas they cover, which on many models, ain't much.
They are a similar price new.
They will need fettling to fit.
They can be hard to fit/refit without a lift.
They can cause damage to pipes that they touch, especially on the edges.
They dent on road surfaces and stay dented so can cause contact with engine parts.
They are thicker, so they are lower than the standard plastic, they can wear very fast if they hit the road as they have no give.
They often don't have the vents cut out that help cool the engine, and they are heat reflective.
Overall, I decided that it wasn't worth the hassle. However, some swear by them. It's a bit of a marmite thing. As long as you are pretty handy on DIY medal work, I think they have a use.
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