Na Bith Mo Riocht Tá!
The merfolk issue has been done and redone here. They were finally allowed as a PC race, but only because of their poor movement. Without that limitation, they're ridiculously overpowered compared to the other playable races (a net +6 to ability scores compared to the +2 other races get, +2 Natural Armor, Swim speed and the ability to breath underwater, and their only drawback is their slow land speed).i really have no problems with any of the advanced races, as long as they don't come from chapter 4.
i don't see an issue with strongtail. a medium creature with a 15 foot land speed is still slower than a darn halfling. medium and heavy armor slows it down to only 10. they are still having to double move to do what most other races of the same size can do in a single move.
in fact, i would make strongtail a requirement for merfolk. spending 6 turns to close up to a foe the human can reach in one is a pretty stupid drawback.
not every merfolk should be forced to be a barbarian, travel domain cleric, or metal/flames oracle. 15 ft move before armor is enough of a penalty. considering most PCs will have 30. that can be a major difference.
i wouldn't even force strix to take clipped wings, flight isn't really any more powerful than walking. unless you are outdoors on the open plains. most dungeons have low ceilings to make flight impractical, and even if you could bypass traps with flight, all you are doing is saving yourself a handful of hit points and happy stick charges. you pay a price in lower attributes and in having less overall racial powers to work with.
I've got mixed feelings about the Strix's clipped wings trait - there are a lot of different adventure types in LPF, and it wouldn't be too difficult to pick and choose those where the ability to fly would be a significant advantage. But as you say, the Strix is pretty well limited in other ways (net +0 to ability scores, limited racial powers other than flight) to compensate.