Just my opinion but sleeve bearings have quite a bit of clearance in them which allows fairly large particles (.001 inch, 25.4 microns) to pass thru.
They usually have a soft Babbitt or lead coating that can also allow larger hard particles to embed themselves and sink down below the surface where they will only cause light abrasion.
Rolling element bearings like ball, roller and needle bearings can be damaged by steel particles as small as .0005 inch (12.7 micron).
Hydraulic valve lifters also use very tight slip fits in their internal components which are less than .0005 inch.
As the Royal Enfield UCE uses rolling element bearings and hydraulic valve lifters the need to have a quality oil filter is very important.
Like most engines that use a good pressurized oil system with a filter in it, the UCE's filter has a bi-pass to allow oil to continue to flow when the filter becomes clogged. This bi-pass in the UCE is the spring loaded parts in the filter cap and they are designed to be used with the right filter.
The unfortunate side of this bi-pass is that it allows unfiltered dirty oil to get to all of the critical components in the engine if the filter becomes plugged.
That's why its most important to make sure the correct filter is used and that it is changed at the recommended intervals.
Saving $5.00 on an oil filter at the expense of a number of $20 parts is a bad idea.
I probably should mention that when I write some of my posts I'm thinking of the folks who are new to working around engines and doing their own servicing.
I figure that if they understand the reasons for some of this stuff it will stick with them better.
I realize many of you guys already know this stuff so please try to understand where I'm coming from.