Add page with drawbacks

docs/drawbacks.md

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+# Drawbacks
+
+Trying to represent STAC data in GeoParquet has some drawbacks.
+
+## Unable to represent undefined values
+
+Parquet is unable to represent the difference between _undefined_ and _null_, and so is unable to perfectly round-trip STAC data with _undefined_ values.
+
+In JSON a value can have one of three states: defined, undefined, or null. The `"b"` key in the next three examples illustrates this:
+
+Defined:
+
+```json
+{
+  "a": 1,
+  "b": "foo"
+}
+```
+
+Undefined:
+
+```json
+{
+  "a": 2
+}
+```
+
+Null:
+
+```json
+{
+  "a": 3,
+  "b": null
+}
+```
+
+Because Parquet is a columnar format, it is only able to represent undefined at the _column_ level. So if those three JSON items above were converted to Parquet, the column `"b"` would exist because it exists in the first and third item, and the second item would have `"b"` inferred as `null`:
+
+| a   | b     |
+| --- | ----- |
+| 1   | "foo" |
+| 2   | null  |
+| 3   | null  |
+
+Then when the second item is converted back to JSON, it will be returned as
+
+```json
+{
+  "a": 2
+  "b": null
+}
+```
+
+which is not strictly equal to the input.
+
+## Schema difficulties
+
+JSON is schemaless while Parquet requires a strict schema, and it can be very difficult to unite these two systems. This is such an important consideration that we have a [documentation page](./schema.md) just to discuss this point.