You've received some very good and accurate responses.
The experience most closely aligned with my own is described here by Pinac whose opinions and experience I greatly respect.
"...with the exception of the BORG, where I can fine-tune the collimation (or should I say: the conditional alignment) right during my observation, so going beyond 100x with the BORG has been a revelation (I have gone up to 125x with 4mm eps and, based on the excellent result, have ordered 3mm DeLites that will allow me to go up to 166x).
So the collimation accuracy argument is not necessarily valid for all BTs."
It is an understandable but unfortunate fact that most manufactured BTs must be conditionally aligned at the factory and can not easily be adjusted during observation by the end user. In designs where the individual tubes can easily and quickly be conditionally aligned during observation the regularly accepted limits on magnification can be achieved, i.e. 200X or more with 4" ED scopes.
When viewing objects at higher magnification you are more likely to be focusing on the image near the center of the field so if the view deteriorates even 50% out from center it is not so detrimental.
Here is an excerpt from a report of a recent outing with a 127mm achromatic BT for your consideration.
"Jupiter was very pleasing at 116X as was Saturn nearby. Everything looked promisingly stable so on to 170x with a pair of Barlowed 9mm eyepieces. I thought no chance but then again nothing to lose but a little sleep so the 9mm EPs were replaced with 6mm using #8 filters.
Now it's operating at 256x, both planets looked very good (not perfect, but I'm yet to see perfect) and Jupiter's moons looked like discs as opposed to the point of a star. No whirls for me yet but nice banding and an occasional hint of some detail. I could see banding (ring shadow?) on Saturn, a tiny bit of planet disc below the ring and Cassini division was sharp.
Just for fun I went back over to the Double Double at 256X which featured clean, tight stars widely split. M57 was a suitable finish to a surprisingly fine evening.
Considering the lenses were operating at about two and a half times what they are designed to support I was very impressed. Merging is simple. I only had to adjust it a total of three times all night typically after an eyepiece change. Each time is was the work of about 30 seconds. There was absolutely no eyestrain even at 256x. If you have "normal" visual alignment it would be even easier."
If more factory built BTs had the mechanisms to allow for quick and simple conditional alignment during use you would see them used at the same magnifications as their monocular brethren.
Two other valid points have been raised.
IPD is indeed critical at higher magnification but if a mechanism for quick and easy adjustment during observation is provided it is the work of a few seconds to set.
Tracking is challenging but only the equal of the same magnification in a monocular telescope. If you have a good mount you will be able to track at higher magnifications.
Oh yes, prisms were also mentioned. In an astronomical telescope? What prisms? (Just kidding, the 90 degree asymmetrical BT design employs one mirror per side.)
I fear I must respectfully disagree in this instance with edwincjones. A well designed binocular telescope is the right tool for the job. ;-)
Seeing planets and other objects at medium to high magnification with two eyes, through two objectives, is a wonderful experience.