Only real difference between oval and rectangular brushes is oval are, obviously, bigger and are less fatiguing to use. That may sound silly to some but when you are doing multiple sets of skis, it's a big deal. Also, with the larger surface area of an oval brush as well as the strap that they have it is much easier to hold the brush flat which means you are using more of the bristles, evenly. Often, people push harder than they need to and this does two things. 1) You'll end up only using part of the brush. 2) It'll bend or splay the bristles. For the brush to be most effective the bristles need to remain as vertical as possible. When you press to hard the bristles roll out and you lose the efficiency of your brush. Think of them like your toothbrush, press to hard, the bristles roll out to the sides, then most of what you have contacting your teeth are the sides if the bristles. Brushing out a ski is no different than brushing your teeth, let the brush tips do the work.
With regard to directional brushes, this has mostly to do with metal brushes, particularly steel brushes. The course, long wire, steel brushes are typically made with the bristles having an obvious slant to them and should ONLY be used in one direction and any other brush that is made with slanted bristles should be used the same way. Some of the stiffer short bristled metal brushes can develop a direction to them if they are used with to much force applied but like I mentioned earlier, if the bristles become bent over or splayed, you're using to much force.
It's much better to use a progression of three brushes correctly than it is to use one with a lot of force.
As for scrapers, they are no different. Keep your scraper sharp and straight and let the scraper do the work. Don't use brute force and try and take it all off in one or two passes. You're better off making multiple passes with light pressure than you are driving the scraper into the ski base and scraping off your wax in one of two passes. The more pressure you apply only dulls your scraper faster and worse, it'll bend or bow the scraper and over time you'll have a slightly concave ski or at the least, less structure down the middle. IMO, it's always best to use a good thick scraper versus an easily flexed thin one.
It doesn't matter if it's brushes, scrapers, files, ceramics, or whatever, always let the tool do the work not the amount of force you apply. If you stick to that, you'll not only get better results but all your tools will last longer, too.