Why are light nodes considered not fully trusted?

[chainofzee]

light nodes are able to check the block header which contains the merkel root and if anything was manipulated, the root would have been changed.
with all that in mind, why do we still consider them not fully trusted?

[ranochigo]

If you only get the block header, then you can't validate the validity because you are unable to validate anything else in the block that is not within that block header. Then, you rely on the longest chain being valid and follow them, without being able to validate the blocks in that chain.

Merkle root can be valid, even if the transactions aren't.