There are a number of problems with digiscoping, I'll mention a few. This refers to small (objective lenses 127mm and smaller astronomical refractors).
1. The scope cannot be used in "prime focus" mode. This is where the objective or front lens set is used to form the image and no intermediary optics. This allows you to shoot with as fast a set-up as is feasible since you are using the front lens at its focal ratio (often f5-7 with modern apochromatic telescopes). This opens up the ability to truly use a scope as at a semi-reasonable speed instead of the double-digit focal ratios you end up with when combining the scope's objective with intermediary optics. Faster shutter speeds, lower ISOs.
2. 45 degree prisms. They do correct the image orientation from left to right, but the prism optics used in them induce optical problems like chromatic aberration and extra internal optical surfaces to cut contrast and definition. A 90 degree mirror diagonal of high quality (as used on an astronomical telescope) does produce a reversed left-right image, but that doesn't effect your ability with the scope once you get used to it. More importantly, unlike prism assemblies, mirrors don't induce any optical errors. Ideally, for best quality should be used straight through with as little optical elements as possible in the optical train. However, you can get 45 degree terrestrial prisms for astro scopes too.
3. Inferior internal baffling. Most astronomical apochromatic refractors have multiple internal baffles to define the light cone from the front lens and produce better contrast.
4. Higher quality optics, in some cases. Astronomical optics are typically corrected to 10x greater accuracy than sports optics and camera lenses. They have to be to support the kind of magnification needed to see details on planets and to control chromatic aberration and residual spherical aberration.
5. Inferior eyepieces, especially the zooms. Astronomical eyepieces, the more advanced ones are more highly corrected than most spotting scope eyepieces and produce better images, especially at the edge of the field. For the highest contrast, simple 2 element astronomical eyepieces such as orthoscopics or plossls can yield the highest contrast and sharpest images.
6. Cost. A good spotting scope will run you $1000. A highly corrected astronomical refractor can be had for $500 with the same diameter front lens. If you want inexpensive yet serviceable, you can buy a Firstscope 400mm f5.0 from Orion in the U.S. an achromatic refractor that will produce nice prime focus shots. It's about $200 and all metal, not cheap plastic. A $200 spotting scope is best avoided.
7. Mountings. Camera tripods suck with long focal length lenses, unless they weigh a ton. There are a number of alt-azimuth astronomical mounts that are portable yet much more stable than most camera tripods and heads. The closest thing from the photography realm are the gimbal mounts, which are also inferior because they have only one "arm" and are not very stable with larger, longer focal length lenses.
8. Flexibility. An astronomical refractor can be used with dozens of standardized eyepieces to tailor the unit to your exact needs. Spotting scope rear optics are all proprietary to the brand.
9. Focusing. Focusing without a course and fine focus mechanism (as is featured on all modern apochromatic refractors) is a nightmare at high focal lengths. 13,000mm is no fun with a focuser that causes the scope body to vibrate at you use it. You can even motorize an astronomical focuser to completely remove human body tremors from the equation. Spotting scopes with large helical focusers can focus faster, but it's easy to overshoot with a camera attached.
People thinking Nikon, Vortex, Leica, should consider brands like William Optics, Celestron, Meade, TEC, Takahashi, Astro Tech, Orion or any of the other brands of high quality apo refractors.
