They sometimes degraded the sharpness and brightness of the image by introducing that extra optical element. The latest versions avoid degradation by using high-transmission lens coatings, plus an additional element or two to improve the correction of the system. Moreover, they maintain the more comfortable eye relief of the longer focal length eyepieces with which they re used. Anyone telling you otherwise is using outdated information. I suspect that so many lenses would just have to prove somewhat detrimental to light transmission.
Most telescopes can use any of the Barlow lenses on the market.
However, the position of the focal plane on some scopes precludes using certain Barlows with particular eyepieces. A Barlow offers the best performance at a single optimal position in the lens system. As a matter of fact, all the Barlow lenses currently on the market are non-adjustable and provide a certain stated amplification factor. If you have an adjustable Barlow, it's wise to experiment to locate the ideal position for the lenses. Despite the specs for a given Barlow, there is actually a way to get more than one amplification factor out of a single, non-adjustable Barlow lens.