I don't get this. Usually FX in recordings are always post cab, and they don't sound "dull".
Here's thing thing with this statement: Many guitarists use a delay or reverb before their amp. Most guitarists don't want to run 4 cables between their pedalboard and their amp. In a live or practice sense, it doesn't necessarily make sense to have delays/reverbs sitting post-stack. In a studio, sure, it's fine, you should probably add your effects in post, but it is a different tone.
For instance, a heavy, cascading delay (or multiple delays) can act almost like a volume boost. Before the amp, this can cause it to start breaking up. After the amp, you just have a louder sound. Depending on the effect you're going for, both are very valid use cases.
Anyway, all that aside, the Kemper does have some good effects, and this is an example of them. They are underrated, as the shining point of the Kemper is the amp profiling process and the quality of the sounds. However, for what I do (ambient/post-rock), the delays and reverbs just aren't big enough. As far as delays are concerned, there aren't enough different types (a bucket brigade would be killer, for instance) and adding delay in multiple spots isn't possible (I like chaining delays to create space). The reverbs... same. We've been waiting on a spring reverb for forever, and the "ambience" sound just doesn't get there for me without completely washing out the original sound. This is why I stick with the Timeline/BigSky combo (stereo FX loop, Mod slot). I generally leave the amp's delay and reverb blocks off. The overdrives and fuzzes... I defer them to my board as well, because my sounds originate from multiple overdrive pedals stacked.on top of one another. The Kemper does a pretty faithful recreation of a TS, but other types of overdrives and fuzzes are missing.
However, I do use tremolos, flangers, compressors, EQs, etc. on the Kemper, and they work and sound excellent. I truly believe that, to this point, Kemper has not involved themselves on anything "extreme," as there are external options for getting those more extreme sounds. It plays like a head unit, and that's excellent for using it in whatever way you feel necessary. For me, it's not an all in one unit, but it works great, and sounds absolutely amazing.