Expertise seems to me to be the equivalent of the Fighter's Fighting Styles. Those are limited to +1 or +2. Yet Expertise gives up to +6.
The big difference is that a fighting style is something which comes into play every round of every combat - often more than once, for most of the styles. A skill to which Expertise has been applied is something which might only come up a handful of times per session; thief tools, for example, are unlikely to be used more than half-a-dozen times per dungeon.
When you compare two bonuses with such wildly varying frequencies, the less frequent bonus should be much greater in magnitude in order for it to make the same overall contribution. If anything, Expertise is too
weak at low levels, where the Archery fighting style applies that same +2 every round. So in terms of a comparison between the abilities, I definitely wouldn't say Expertise is too good.
The issue comes with Bounded Accuracy, though. As a design goal, Bounded Accuracy is supposed to ensure that you never
need to hyper-specialize in order to compete - there should never be a case where
only an optimized character has any chance at all, and where someone who actually invested resources in something could still have zero chance. From that perspective, Expertise is definitely broken, although it shares some of that blame with Reliable Talent.
In my game, Expertise gives you an extra 1d6 to roll, and Reliable Talent only kicks in if the sum of that 1d20+1d6 is still below 10.