Are the bluetooth,wifi, brightness problems in android devices are related to the ROM or the hardware?
They can be either. If you're running an OEM ROM then it's more likely to be hardware. If you're using custom ROMs, especially nightlies, it's more likely firmware although never rule out hardware.
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In theory, yes. Although I tend to stick to flashing through recovery. There's always a chance you will brick your phone but if the process is done correctly then the chance should be minimal.
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Recovery, for example, ClockworkMod Recovery is a step in and is created through in the process of rooting.
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Lots of support for older devices such as S2 and especially S3. Not so much since then besides Note 3. Should see more as people's 24 month S3 contracts run out and turn to newer devices.
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