A chain oiler makes sense on a Motorbike as they will fling oil off the chain, seeing as the chain is moving far faster so steady metered lubrication makes more sense, for an MTB all you will be doing is adding oil to the already contaminated greasy shite on your chain…
Probably just better off taking the time to clean and re-lubricate your chain properly if you want it to last…
as for cable oilers never had a a problem with either sectional outers or a full length run; wipe clean at the ends from time to time, apply a few drops to the cable, shift back and fourth a few times, then wipe away the excess… But I guess cable oilers won’t do any harm if you fit them so why not…