both authors told me the STM32 is TOO difficult for a MCU beginner to learn, like me. But may be ok for you.
They are probably right! I had enough problems getting up to speed on the STM32 board I bought a few years ago that I just put it aside and moved to the original mbed (LPC1768) and I had a ton of experience with uCs and programming.
I concur - if you are a newbie only getting into MCU programming, then starting with ARM straight away is learning to run before you have learned to walk. Those MCUs
are complex!
In that case I would suggest sticking to an 8bit micro for a while longer so that you learn the ropes first. Even that could be overwhelming enough already when you don't have 5+ registers to configure to just blink a LED like on ARMs.
STM32 micros are powerful, but with power comes complexity. The Cortex M core is not too bad by itself, but if you were used to something like AVR or a PIC where a peripheral is typically controlled by two or three registers, you are in for a rude shock.
The reference datasheet for the smallest F0 series chips is around 1000 pages already. For example, on STM32F042 (a small USB-enabled micro), each GPIO port is controlled by ~11 registers + additional registers controlling power to the port, interrupts, DMA, clock ... Fortunately you don't have to configure all that every time, most often the power on defaults are sane for the common situations, but still. Compare that with about 3 registers you have for GPIO port on AVRs or PICs. And that is GPIO - a fairly simple thing. You don't want to see stuff like timers or I2C ...
The various libraries (HAL, STDPeriph, mBed, ...) abstract this away and hide it behind layers of code. On the other hand, when the function you need is not handled by the library or the chip is misbehaving because of a library bug, you are going to have fun ...