Well! As you might guess, I must respectfully disagree with your that assessment. I think Martin is actually not a bad character-writer, but his worldbuilding is actually quite shoddy if you look beneath the surface. If I were to explain in detail why (and why it certainly pales before Tolkien's) I'd be here all night. So instead I will try to summarize the most egregious flaws in bullet-point form:

• Martin can't seem to make up his mind on what his armies look like and how they operate. You have Septon Meribald's story about how he & so many other soldiers were dirt-poor draftee/volunteer schlubs fighting with kitchen knives & farming implements, but the armies we actually see duking it out are capable of standing their ground in the face of massed cavalry charges and executing complex maneuvers (ex. the Battle of the Green Fork near the end of AGOT). Historically drafting peasants was an absolute last resort and most battles were fought between armies of knights, men-at-arms & mercenaries, because nobody in their right mind wants soldiers who can't fight and who you need to work your fields anyway. GoT armies act like the larger and more professional armies of the 16th century onward, up to & including tearing apart the countryside and terrorizing peasants in their way to 'forage' for supplies.

• Martin's armies are in general far too big for such a 'realistic' medieval setting, even a late medieval one. Take the aforementioned Battle of the Whispering Wood: the Lannisters have got 20,000 men (including 7,500 cavalry!) while the Starks are fielding 17,000. The Tyrells and Renly field a host of 100,000. In reality medieval European armies were quite a bit smaller, being comprised (as said before) largely or entirely of knights and their men-at-arms who were typically only obligated to serve for 40 days; after that their lord would have to start paying them, which almost nobody could do because outside of the Byzantine Empire, most feudal realms collected taxes in kind rather than in gold & silver. Also, medieval logistics could not support such large armies for long. Tolkien comparison: the good guys have almost 15,000 men between them (8,000 Gondorians & 6,000 Rohirrim) at the Pelennor Fields, the climactic battle of ROTK.

• The Ironborn make no sense as a culture. They're reavers who have terrorized the mainland for millennia...despite hailing from a bunch of islands consistently described as barren rocks with little in the way of vegetation or arable land. Unlike the RL Vikings they're ostensibly based on, they actually HATE trading and aggressively look down on any of their kind who pay the 'gold price' (trading abroad) instead of the 'iron price' (raiding for shit). So where do they keep finding wood to rebuild their navy, which has been not only defeated but completely crushed on several occasions in history? For that matter, why haven't the continental powers who dwarf them completely crushed them for being nuisances and settled the Iron Isles themselves, despite said isles being conquered by mainlanders from time to time? How did they manage to conquer and hold the Riverlands, a much bigger and more heavily populated kingdom than their own, for centuries despite a long history of hostility & stark differences in culture & religion between the two? Tolkien comparison: the Corsairs of Umbar are a pretty advanced civilization descended from the Black Numenoreans and didn't spend all their time being annoying pirates.

• The Dothraki make even less sense. They're clearly based on the Eurasian nomad hordes, but don't wear armor, don't have bows for their iconic weapon, have no fixed settlements at all besides Vaes Dothrak, and disdain settled civilization utterly. Their usual tactic is to just frontally charge at their enemies over and over until one side breaks, most hilariously attempting this against an Unsullied pike phalanx 18 times in the historical battle which made the latter famous. They should not realistically pose any threat to any non-Stone Age opponent, unlike the real Eurasian hordes which were often comparable to or even more advanced than the settled civilizations they battled and whose political practices could even make their rule more attractive than the kings they displaced (ex. the Huns and Mongols). Tolkien comparison: the Easterlings, and particularly the Wainriders, were noted to be extremely dangerous opponents to Gondor thanks not just to their barbaric ferocity but also to their excellent arms & armor and innovative tactics (involving horse archers & chariots).

• The Faith of the Seven, the main church of Westeros, has comically little power for a 'realistic' medieval church. The Westerosi have religion but they don't actually believe in it, much like post-modern people these days tend to. Meanwhile actual Medieval kings were highly faithful and served a role in worship. They built edifices to the church and things like a King being unfaithful to his wife were huge problems that could topple a kingdom. If you were excommunicated, the people believed that working with you meant they would be going to hell with you later, and they believed it; just look at Holy Roman Emperor Henry IV, whose excommunication left him kneeling in the snow & begging the Pope for forgiveness for three days because literally none of his lords and servants would lift a finger for him. Tolkien comparison: the Free Peoples don't have organized religion but they do have a fairly pervasive sense of spirituality, and good reason for it (the last time they had a temple & organized clergy, both were puppets of Sauron) unlike ASOIAF, where the Faith inexplicably never tried to regain its power in the nearly 200 years after the Targaryens lost their dragons until the High Sparrow arises in AFFC.

• Lack of cultural variation among the Andal Westerosi kingdoms. There's no way 5 out of the 7 kingdoms, which comprise the majority of the continent's kingdoms and population, should be basically interchangeable and all speak the exact same language, even despite their shared Andal ancestry and worship of the Seven. The Roman Empire had more regional variety than that, to say nothing of just medieval Western Europe or the HRE alone - without touching the Balkans or Eastern Europe. Tolkien comparison: the Gondorians had regional and cultural differences between themselves, with a Sindarin-speaking Numenorean elite lording it over the Middle Man majority (which only grew as the blood of Numenor waned) and Westron developing later as a mutually intelligible universal lingua franca from the former's original Adunaic language, but even that wasn't without its well-developed regional dialects such as Hobbitish.

I could go on but I'm already halfway to the .win character limit. To sum it up, Tolkien's worldbuilding isn't flawless - I don't think anyone's is, and there's always room for improvement - but it is insanely ahead of and certainly far more consistent than Martin's, which (as I have basically said before) isn't so much the realistic medieval world it's been advertised as, as it is a post-modern progressive undergrad's understanding of medieval Europe through pop-culture-tinted lens. ASOIAF isn't without its good points, but I would definitely not count the worldbuilding among those points, and instead argue it's one of the weaker parts of the work instead.