There are a number of trade offs to be considered when choosing focal reducers. For example:
Focal reducers make the image size smaller. This means that the image of a given object will fit on a smaller sensor. It also means that each pixel will receive more concentrated light. The net result is that field of view is increased and exposure time is decreased (photographic speed is increased). These are both generally considered to be good things in the EAA world. The trade off is that reducers steepen the light cone, increasing the angle of the light rays. This magnifies image train misalignments in collimation, focus, sensor tilt, mirror tilt (flop), etc. It also increases coma. The bottom line is that it makes the image train "fussy" and less forgiving.
The better SCT focal reducers are also field correctors; for example, the common Meade/Celestron 0.63x (aka 6.3) SCT reducer/corrector. These will not only "reduce," they will also flatten the image for use with photographic sensors, therein, improving image quality. The tradeoff is that they typically include 4 active elements and sometimes a 5th passive element. That means the light must pass through 8 to10 glass surfaces. Each glass surface reduces the light transmission, and degrades the image, by causing internal reflections. Stack two of them and you have a lot of glass surfaces in your image train. Additionally, stacking two of them means increasing the correction as well. This can lead to an over corrected (distorted) image. The strength of this effect varies with the spacing between the correctors. I haven't been happy with the results I have gotten when stacking reducer/correctors, but other CN members have experimented with this and found combinations that they like.
Reducers allow the use of a smaller imaging sensor, which reduces the camera cost. The tradeoff is that reducers aren't free. Plus, you need to consider the cost of spacers and the time spent experimenting and the potential reduction in image quality. Instead of using a reducer, you can achieve the same (actually better) net result by using a larger camera sensor with larger pixels (or binned pixels). Where do you want to spend your money?
As a general rule, my experience has led me to prefer larger camera sensors over reducers. I prefer to minimize the amount of glass in the light path. And good used astro modified DSLRs (large sensors) are becoming less and less expensive as people flock to the latest and greatest astrocams. But DSLRs have their tradoffs as well.
And to be clear, there are good exceptions to every rule. For example, the common 0.63x SCT reducer/corrector was designed specifically for photographic work with f10 SCTs and has stood the test of time. It is a no brainer. In another example, I use an ASI290 mini mono with a cheap 0.5x reducer spaced for 0.8x focal reduction in a 50mm e-finder. In that case, FOV and speed are more important than image quality. Both of these examples are near perfect uses of focal reducers.