Mature
How much can they improve on an CPU core that is 25 years old.
Except for new releases they should have the bugs worked out.
The IDE is a separate product, they are just sold together.
It is expensive, but it works. Some of the competing products I am sure are good. Not that I have tried them.
$100 dolar compilers have limitations. I am sure if they could sell them for Keil prices they would. Have I used them, No. Have I heard bad things about them , yes. Open source, Is open Source. Find a bug, fix it and send it in. It does not optimize to you liking change it. Does not support your special core Add it.
Good deal if you want to work on the compiler too.
If not, nothing beats rewriting you code to trick the compiler in work right at 2 in the morning.
Fining shills for Keil would be easy. Is it perfect in all realeses not. But a good product.

BTW: check the Keil fourms for compiler bugs. Most are answered by RTFM, No That is the way C works, or Post you code.