I just had to do the heater core and rad on my F-150 this weekend just past. I have done both of those in my mustang in the past. Working on my truck really makes me appreciate how easy my old car is to work on.
Let's just say I never had to this while working on the mustang's heater....
...or this....
Something I have noticed on the board here is that the guys who have souped-up engines are the ones that complain most frequently about repairs or breakdowns. Well, what do you expect when you are trying to get 1 to 1 1/2 hp per cubic inch? Breakdowns are inevitable.
My mustang is my primary vehicle, the 4X4 is for my wife and young son. My motor has a mild cam, Edelbrock intake and 600cfm carb, pertronix II, headers and 3.50 gears. I have no dellusions of the hp output, but I know it is better than stock. I haven't had a breakdown or had to have it towed ever in all the years I've owned it (other than a problem with a tire once).
The point is that the compromise for higher hp output is reliability, you just have decide how much you are willing to compromise and accept the results.
If you want both power and reliability then go down to your Ford dealer and buy a brand new mustang.
I feel for you, but it is the nature of the beast so to speak.