**This response is speaking in relation to lite duty motors (3/4, etc)**
Sealed motor bearings have been in use for some time and actually, generally have performed quite well. Of course there are times when they fail as does the serviceable motors.
Those in engineering have found that when looking at all factors, oil less or sealed motors last longer. There are a number of reasons behind this...mainly the huge volume of misunderstanding by service technicians in regards to proper oiling. Many times techs over oil, leave out the plugs allowing contaminants (rain on outdoor units or condensate spray on indoor units) to enter bearings sleeve, and use of improper oils for lubrication.
Obviously, there are those who utilize and service the serviceable motors properly but there are far more that do not. This causes a much larger rate of failure in those motors with the "unsealed' bearings.