There are hundreds of varieties, some juicier, some more tart, some meatier. The biggest thing about heirloom tomatoes is that diversity in crops is very important. The other is that they're beautiful. So you get this variety of tomatoes of all colors and shapes, and some are tart, some sweet, some more meaty than others.
All tomatoes, heirlooms, beefsteaks, plums, should be completely ripe for a salad. If they're not, it's not the same. There is also no comparison between a local tomato and a tomato from thousands of miles away, and that has been kept in storage.
The next one is that tomatoes should not be refrigerated unless it can't be helped, and they must be room temperature for a salad. Cold tomatoes are pointless.
Now the really big one is that tomatoes need salt, and a good amount of it. Lots of cooks really need to push the salt when making salads, but tomato ones in particular. It really does make the difference. Tomatoes and salt are made for each other. If you can salt them for a little while to draw out the juice, so much the better.
But basically, they're still tomatoes and they don't taste dramatically different from any other tomatoes that are also ripe and juicy.
If your tomato salad didn't have warm, bursting with juice, perfectly salted tomatoes to create the perfect balance of sweet, tart, and savory, then I can see how you would find it lacking.
To me it sounds like between the yogurt and feta there's a bit too much creaminess going on and perhaps the sweet balance needed adjustment.