Amiga 600 the worst?

[Minuous]

It was the low-end member of the Amiga range for a while, so I suppose it was "the worst Amiga model that you can buy" new at that time. The same could have been said of the A500, A500+, A1200, etc. at other times.

But certainly a decent machine. The worst Amiga in terms of capabilities would be the A1000. In terms of price/performance, I would propose the A2000: an extra thousand or two dollars (compared to A500) basically just for a few card slots.

[Minuous]

Many of the old games failed to run on it and I guess that's why it's been said as the worst model.

The same applies to every single Amiga model ever except the A500. Properly written games will work, badly written ones may not. It's not fair to blame Commodore for lame third-party game programming techniques.

[Minuous]

Wasn't the whole reason for compatibility issues due to a 2.X ROM?

Yes, that's the reason for the vast majority of the compatibility issues.

I always wondered if a ROM switcher with an added 1.X ROM would bring better compatibility to the A600...

Yes, or you can use eg. ReloKick to soft-kick it.