I have only recently investigated this feature of ODK - using Google Sheets as a server in place of ODK Aggregate. Not sure how I overlooked this when first learning about ODK, it seems really helpful for people to try things out without having to install a server, and as a feature it seems to have been around for quite a while! Another pleasant surprise of ODK...!
I enjoyed making this video about ODK Aggregate and was planning to do another about using Google Sheets instead - even faster for new users to get started.
In terms of features it seems the main drawbacks are: "Ensure no repeats, audio, or video prompts are in the form. Groups, pictures, GPS prompts are perfectly fine." - you can take pictures and upload these, but you can't use images in the form (as there is no media upload step like there would be with aggregate). You can't record audio or video.
A different type of consideration for me would be that being based in the UK it is useful for me to be able to use App engine to determine where my project server is based (in the EU for data protection reasons) - by using Sheets instead, I wouldn't know where the data is physically stored. Although that wouldn't be a concern if you weren't collecting personal data, depending on your deployment.
I would be very interested and appreciative to hear of any further practical insights, technical concerns or perhaps even legal and ethical reasons you would consider when looking at deployments based on Google Sheets or ODK Aggregate. Thanks !