It's hard to analyze a problem with no pictures, and little description beyond yellowing and browning, so I'll have a go at a couple of general observations about yucca. When people have problems with them as indoor plants, the usual reason is the soil staying too wet. Especially if they have light any less than a bright south window, the soil needs to get almost dry, to the bottom of the pot, between waterings. Do you check the soil with some kind of probe or stick to make sure it's not too wet? On the other hand, keeping them too dry will also cause problems; testing the soil will also help you avoid this mistake.
Scale are not so common on yucca, mealy bug are more likely; how do you know you have scale? Regardless of whether scale or mealy, you can control them by spraying plant well with a soap and water solution - use 1 tablespoon of mild liquid detergent in 2 cups water, spray plant on both leaf surfaces and into the crevices between leaf and stem - spray to dripping, once a week for 3 weeks. If you want to get creative, add 1 teaspoon of alcohol. Of course there are a number of insecticides you can use, but I personally prefer to go it without poison.
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