Originally Posted by FBinNY

Two points to consider.

1- you absolutely cannot mix angular and radial bearings on the same axle. Angular contact bearings require a thrust load to maintain contact, and the reaction forces would be harmful to a radial bearing which isn't intended to handle significant thrust forces.

2- Angular bearings require micrometer adjustment to establish proper preload and zero play contact. Many (most?) hubs using radial bearings lack that kind of precise adjustment capability, and likewise cannot be set for an axial preload.

From a practical standpoint, there's no benefit to angular contact bearings other than the fact that hubs designed for them have the precision adjustability (beause they have to), while many hubs designed around radial bearings lack it. OTOH - if your hub has that level of adjustability, then you can use it to eliminate any play on the existing bearings, so a change isn't necessary.

FWIW - the most significant rational for choosing angular vs. radial bearings in bicycle hubs relates to design and manufacture, and not to function Angular contact was the logical choice for years because they made sense when using threaded axles. The user fine tuning that design offers frees the maker from manufacturing to precise tolerances. It's a rude/crude design, easy to produce, and easy to maintain, and has served for over a century. Radial bearings require close tolerance manufacturing, unless some user adjustability is provided. With modern manufactucturing methods, the required precision isn't a problem, freeing the maker to use radial bearings, and saving the costs and weight that the angular contact designs imply.