It would be insightful if you could pin some critical data to understand its structure and post the workflow using the </> button. It would make attempts to help here out less of a guesswork.
Here is how: Google sheet: data is replaced in the existing row instead of adding new row - #8 by Olek
If you want to avoid things like [undefined] appear when a specific field is only occasionally present in the output you can use expressions like {{ messages.content.whatever ?? ""}}. This will efectively add "" when a data point is not available.
[Object Object] appears when you try to put an object (a structure with fields) into a context where a string expected. This how Javascript deals with those situations. The good point to start is to use expressions like {{ JSON.stringify(data.field.whateverObject) }} to understand the structure, problem and if the output with special characters in it is deemed better than having nothing (i.e. [Object object]).
You may also want to use the Structured Output Parser node/tool:
If you find anything above helpful, please mark this post as a 
Solution.