They're blatantly OP in Skyrim. There's no (gameplay) reason to play anything else, if you're running a mage.

How are they OP? I'd argue Breton magic resistance is more OP than +50 magicka. It's easier to pick up an extra 50 magicka either through leveling or enchantment (or make magicka irrelevant) than the magic resistance. But either can be done without too much effort so I don't see how any race can be considered OP in Skyrim.

^This

Seriously, compared to the earlier games race has such a small difference to gameplay in Skyrim that you can play what you want without feeling gimped.

One of my favorite playthroughs was as an undercover Thalmor sent to spy on the Stormcloaks. When Tullius ambushed the Stormcloaks my character was taken prisoner. Feeling confident he did not fear the silent ride to Helgen, he knew the Dominion would set him free. Once the gates of Helgen swung open and saw Elenwen with Tullius he smiled, though when the cart stopped and Hadvar was calling each prisoner by name, he wondered why the Thalmor did not intervene. With his confidence now wavering he took his place next to the other prisoners. After the execution of the first prisoner, my character began to panic. When his name was called next he almost had a panic attack. Why weren't the Thalmor intervening? It was at that moment he knew he was betrayed and accepted his fate. He stepped up to the block. knelt down and put his head down, with eyes closed he waited for the blade. Then when Alduin attacked he saw his chance to flee. He first followed Ralof, then he joined Hadvar. He knew the Empire would be more tolerant to a now ex-Thalmor Justicar, than the Stormcloaks. From there he went to Riverwood and started the main quest. Eventually he joined the Empire, cause he knew the Empire was Tamriel's only chance against the Dominion. Plus he had valuable intel on the Thalmor. Along the way he killed many Thalmor and took the fight against them.