Neither one or the other are bad by themselves, and in some cases it is better with maintaining the versions separate and sometimes no. As you see, a classic: "It depends" situation… However, if the client is starting its expansion in the international markets, then I tend to suggest the domain with language and/or country version subfolders. Doing so, we won't split our technical efforts in too many websites, and we can take benefits from the link building campaigns of each version for every version through internal linking.
Remember, in case of multi-country, that you can geo-larget subfolders via the old version of Google Search Console (which is still alive). Only once we have enough data (and do not forget non-SEO business reason), only then we can start thinking if it is opportune to migrate one or more version to a separate domain name. However, there are cases where you must start with separate domain names. For instance, if you are in the pharma industry, the legislation can be very different from country to country, that you cannot have 1 site only. Another case is when the brand is already present with local offices or shops in the country. In that case, it may be better to start directly with a dedicated domain name for having a unique local message and tone of voice. I like how Apple does it. Only the most relevant markets have a full version of the website with the store. The less relevant no (just a page to local retailers).