Currently there are some problems with getting List Item data from API for Field Customizer: event.fieldValue that is available in onRenderCell event sometimes contains not what actually expected or what is not enough.
For example, Hyperlink or Picture field has 2 properties that should be available in Field Customizer: url and descripbtion, but only url is available now.
Of course, most of the needed values could be requested via REST but it would slow the rendering for the column significantly.

Given that the information is already preloaded on the page, ideally this information should be exposed via an SPFx TypeScript API. For example, the SPListItem class from the page context could expose this data.

Here are some scenarios that require additional logic/requests:
1. Hyperlink or Picture fields - only URL value is available; Description (text for the URL) is not provided in the API. Also, no information if the url should be displayed as a picture or link.
2. Currency - event.fieldValue contains formatted text based on Web Regional Settings. The actual numeric value should be requested separately or parsed based on Regional Settings (which should be requested as well).
3. Number, Percent - similarly to Currency, event.fieldValue contains formatted text. Also, there is no information if the value is displayed as percent. The only way to get this info is to request SchemaXml and check Percentage attribute there.
4. Friendly text for Date and Time - for now there is no (as I know) any endpoint to get the Friendly text that is displayed in the cell (for example, "About a minute ago"). event.fieldValue contains a datetime string for this type of field.