I have been to Lefkada and Lemnos (both last year).
The spots in Lefkada are detailed here: http://www.alwayswindy.com/blog/lefkada-the-kite-spot/
Drove around all the spots in Lefkada but was unlucky with the wind (1 day of marginal conditions only). The most consistent beach is Agios Ionis. Go up to the windmills on the beach shore for the best wind. Its thermal wind so relies on the heat differential on the big lake between the beach and the town. Yes its crowded. You can't avoid it unless its properly windy as its about the only place that regularly gets wind. Lots of learners so just ride upwind and you'll find loads of space. Most people seem to hug the beach for some reason.
The wooden bridge spot is beautiful and looks amazing. The sand is covered in litter though and lots of sharp shells in the water so wear something on your feet. Take a bin bag and pick up litter to take away at the end of the day and it will eventually clean up if everyone does it. I'm told everyone stays at the top end nearest the bridge so its easy to walk down the beach 100m to find a bit of quiet. This is the spot that you will want to kite - but I don't think it gets wind that often.
Lots of people camp out by the side of Agios Nikolaos beach. Can't say much about it though as no wind to work out how it worked.
Looks like a good place for hydrofoiling. Most locals seemed to have big kites (12m+) so it seems like it doesn't often blow big. Water conditions at Agios Ionis were average chop so no better than my local beach. Lovely place but I'm not desperate to return as the wind/water conditions weren't amazing - though I might have been unlucky.
Lemnos was great. Flattish water at one end of the bay, rolling chop at the other. I found the schools friendly although I have read about others who had a less welcoming greeting. Launch point near the schools is about as sketchy as you'd deem acceptable (swirling wind, lots of weed on the edge of the shore) but seemed consistent once you were 50m from the beach. Wind forecast also seemed accurate here so it was easy to work out when to turn up. I'd happily go back again.