Coma correcting eyepieces can provide a lighter, cheaper, more convenient solution that can optically outperform a coma corrector plus standard eyepiece.

Below some of the drawbacks of using standard eyepieces in a coma corrector that are improved by coma correcting eyepieces.

1. Size and weight

Popular coma correctors like the Televue Paracorr 2 and Explore Scientific HR are bulky devices; they are typically about 140 mm long and add around 0.5 kg at the eyepiece.

Furthermore, the Paracorr 2 extends 75 mm inside the focuser. The author's 25" f/5 telescope has been optimized for small secondary size (100 mm, giving 16% central obstruction) with a low profile Moonlite focuser mounted as close as possible to the optical axis. The Paracorr 2 then intrudes in the secondary cage and, sadly, prevents me from using a filter slide on this telescope.

It's worth emphasizing that well designed coma correcting eyepieces do not have to be bulkier than standard eyepieces!
To make the point, the image below compares the Televue Paracorr + Nagler 31 combo to my 28 mm 86° coma correcting eyepiece design. I superposed the 28 mm 86° lens layout on a photograph of the Televue combo.
While the compact 28 mm would weigh less than half of the 1.35 kg of the bigger combo, it produces a 15% larger true field with excellent edge correction, comfortable eye relief, and minimal distortion and curvature.

2. Cost

Televue Paracorr 2 pricing is around $550/€700 in US/EU, Explore Scientific HR around $400/€350 in US/EU, the integrated Feather Touch SIPS adds about $850/€1000 in US/EU.

Money that could have been spent on eyepieces!

3. Barlow effect - True Field of View

The coma corrector provides a slight Barlow effect which reduces the maximum field of view. The coma corrector limits the widest field of view of the telescope, and for me personally this is one of its biggest drawbacks!

Two real examples:

• In the author’s 25” f/5 newtonian telescope the largest field of view of about 40' is obtained with a Televue Nagler 31. In the Paracorr this eyepiece effectively becomes a 27 mm eyepiece, reducing the true field by 15%: the 42 mm field stop of the 31 Nagler effectively becomes 36.5 mm.
  My 32 mm 81° coma correcting eyepiece design would deliver a 45 mm field stop, producing a 49' field, nearly 25% larger than the Nagler 31 combo. This difference is illustrated in the beautiful photograph of the Orion Nebula made by Patrick Cosgrove.
• In the author's 43" f/3.6 newtonian telescope the largest field of view is obtained with a Televue Ethos 21 plus Paracorr. This delivers 18 mm focal length with an effective field stop of 36.2 / 1.15 = 31.5 mm.
  My 20 mm 100° coma correcting eyepiece delivers a 35 mm field stop, producing a 10% larger field than the Ethos 21 combo, with excellent edge correction in fast Newtonians.
  Or use this 24 mm 86° coma correcting eyepiece with a 36 mm field stop, increasing the largest field of view by 15%!

4. Ease of use - Focusing

The Televue Paracorr contains a helical focuser (a.k.a. tunable top) to keep the coma corrector at the right position. When changing the eyepiece, the eyepiece has to be moved at its approximate focuser position with the tunable top. Then you can finetune with the actual focuser of the telescope.

While many observers including the author have got used to doing this, it still is a hassle compared to normal focusing, and adds some mechanical flexibility to the optical chain.

The Feather Touch SIPS product page explains the advantages of not needing a tunable top:

“Once it is set-up, there is no need to fumble with locking screws, turning your flashlight on to check its position, etc. This saves time and hassle. Also, the assembly is more rigid and flexure is greatly reduced. The SIPS Coma Corrector totally eliminates the need to re-adjust/calibrate when switching eyepieces. You will simply adjust the focus knobs of your Feather Touch focuser for viewing. Because of this, the SIPS Coma Corrector will save you valuable observing time, less hassle and ease of use.”

Obviously, living without a coma corrector altogether provides the same benefits, and more!

5. Vignetting

For eyepieces with a large field stop, the coma corrector can vignette the outer parts because the 2 inch corrector inserts a 48 mm diameter lens quite far from the focal plane.

A f/4 cone at 88 mm of the focal point is 22 mm wide, which implies that a 48 mm lens could only provide full illumination to a 26 mm field stop. In practice this is not as bad as it sounds, very often the secondary diagonal mirror will be the limiting factor for the field illumination.

Still, the absence of a long coma corrector in front of the actual eyepiece means that a coma correcting eyepiece can better exploit the available 2" focuser diameter - as demonstrated by the 32 mm 81° coma correcting eyepiece which delivers a 45 mm field stop.

6. Focus in-travel

Coma correctors require some extra focus in-travel, typically 15 mm but sometimes more (the lower the Barlow effect of the coma corrector, the larger the extra focus in-travel will be). This moves the focal plane farther from the main optical axis, requiring a bigger secondary mirror or reducing the fully illuminated field.

7. Number of optical elements - Transmission and scatter

The coma corrector inserts 4 or 5 elements in the optical path, adding to the 6 to 10 elements from premium eyepieces. Every element generates some light loss (glass transmission and surface reflection) and creates additional scatter.

Summary

In short, coma correcting eyepieces can provide a lighter, cheaper, more convenient solution that can optically outperform the combination of a standard eyepiece with a separate coma corrector.