Dissecting Bad Grammar

The second sentence is structured in such a way as to encourage an initial parsing attempt that is actually invalid. (In other words, it is a "Garden Path Sentence".)

The phrasing of "Small projects can act as a sandbox where ..." invites a natural reading that the phrase headed by where is about to tell you what you can do in the sandbox. As you progress, however, "where appropriate to allow you" clearly cannot describe a property of the sandbox; this forces you to stop, backtrack, and actively work to parse the sentence correctly despite its misleading formation.

This structure pretty much guarantees a break in the flow of the reader, a loss of concentration, and a transfer of focus from what is being said to how it is being said, all of which are negatives for a passage that is trying to communicate effectively.