hi potential buyer of Alza 
Don't be afraid of all problem you can read internet. Every car has it problems. Some ppl don't mind certain things.
And you pay what you should get.
For me, Perodua or Proton also got problem. If I think & compare both problem. I rather choose the Perodua Problems, so I chosen Perodua Alza.
U ask the Raining hit the roof sound. Yeah, a bit noisy compare to other cars.
My experience is this:- When raining, I feel that the Front Glass is even more noisy, alrd overlap the Roof Noise alrd 
Just switch ON the CD Player & raise the Volume, no more Roof noise. My memory counted, 5% of my driving is Raining. So I don't mind this.

Wind Noise on Driver Side/Front Left Passenger Windows, complaint when you do 1st service.
Soft Car Horn, I usually don't horn ppl. And I'm not impressed if I have nice sound of horn. So I don't mind this.

For Power Performance. I no longer drive like my first car (Manual). I am normal driving type or slow type. I won't rush to somewhere cause it is dangerous 

Others problems not available for comments cause not in my Alza.
A lot of Good points for using Alza, only it doesn't appear in Internet / Forums. If appear in Internet / Forum, mostly is negative / problems.
For me, got many good points, sometimes also lazy to write about it. If problems, very hardworking to write the whole essay expecting someone to solve the problems.

Okay, one very good points for me is -> comparing few brands & price, Alza good for me is "Practicality" & "Spacious".

Good Luck