Better im start with that one supposed to be at the end.
... But im not say here for OpenXCom this one are bad. it is good remake. I say it should be examined how accurate TftD supported in current versions. Again, this time - GOG bring up-to-date (windows native exe from about '05) version for the game, without need of using separate installer or disc images from GOG (or 3rd party sources or remakes) (hided in game folder, same exe for Xc1 was missing here). If OP here ask, he probably(?) do care what files included even if there is remake.
It isn't relevant
what version GOG ships, as OpenXcom renders all this version faffery moot.
Not relevant at all?
Well, at some point yes. Definitely yes for Xcom1, cause GOG didnt provide latest version alternative. But sharing same engine require a bit more work from OpenXcom devs. When i last time try it, OXC almost just start to support TftD and use non-original battle maps scale (higher than XC1 less than XC2).
Or it be something else... hmm... Not matter. Such remakes never be 100% original gameplay experience: 50% for improvements, 50% for for non-improved differences (new AI). It is something devs should be interested to achieve, and since open-source someone enough interested to exam code for such details as how it be in TftD if now they share same engine. For open-source it is sometimes take decade to fix something implemented 10 y. ago.
It is not so accurate answer like 'who cares what is come with the game, if there is OpenXCom'. Obviously there is old bugs, and there is new ones. Thats axiom. You choose between them.
So first you better get vanilla game in most full\up-to-date form. For such releases GOG sadly not a best source usually.
If you need more examples of that theoretically thing (mean: obvious), i cant provide enough for OXC (and it is about old time when TftD just added, im not play it after), but can provide for Theme Hospital CorsixTH. Good one example, not all features still implemented, existed implemented in few very last versions, there is issue with them (new feature for random earthquakes implemented before campaign-scenarios based, now both are included and calculated by default and you need tweak this or you get too much earthquakes in campaign). Also good example of native Windows exe which one not available on GOG like for most DOS and ScummVM releases here. Like X-com 1 here too.
Another good example is Warzone 2100. It have many issues for decade. Some 10yars bugs never be fixed. Some new features broke original and some version even remove old unit controls system features (while it is readded, there is a chance they not work properly). New bugs (auto-rapair-turret not repair automatically). And new contributors who not interesting or just cant fix what need to, so they bring campaign balance changes. Which ones by devs word supposed to stay original with all changes to MP only (and never be like that, Wz2100 always share balance).
So OpenXCom is have general issue - new AI. Which one can be better or not. It is just differ. Xc1->Xc2 engine original improvements not be respected first TftD support been added. Few implemented as improvements for Xc1 already, but that require review how it in fact applied to Xc2 now. it took time for such projects. Sometimes literally decades.
All that irrelevant. But there is ppl who still try to argue with that, with that im not wrote here. Original is original. Remake is remake (official or not). They never be same, what is obvious, so obvious = give differ experience = they should both be played. Remake wont give you original exp, original obviously not have new remake experience.
Sometimes that bring more issues when new contributors not even respect original experience at all, which one broke core ideas - current versions of Wz2100 campaign. When OpenXCom or CorsixTH is good remakes (for CorsixTH even with missing features), this one more like bad remake for campaign, even if close eyes on many else technical issues. Sadly, it be better during v.2.2.2 times. ...