Players that're optimization-minded will get their ACs up to silly levels, especially if all the splatbooks and multiclassing etc. are allowed. But even with core book stuff, it's quite easy for a tanky PC to get their AC up to the mid-20s in later tier 2 play. Obviously that's not going to be only-20s-hit, but toss in a few spells from teammates or additional frequent-use items and you're hoping you can threaten them with other methods. Unfortunately I've had situations where well-regarded 3pp or other 5e products make the situation even worse, which has caused me to tighten the reins on what content I allow in by default.If bounded accuracy is working you shouldn't be able to get your AC high enough to be too much of a problem. In the 3e day's it was certainly possible to focus on your AC to the point that only a 20 hit you, but if bounded accuracy worked (and it seems to have) your enemies should still have a decent ability to hit you.
How high have you gotten your AC?
On the design-side, bounded accuracy hits trouble with those not-so-edge cases if the DM is trying to play with default monster math, partially because monsters rely on CR for their to-hit.

