Although it is convoluted to explain, the process of moving the ASI to character, rather than class, levels is easy to follow and doesn't cause any real problems. Most problems it causes have an obvious solution that allow us to bypass 'dead' levels.
Example:
Level 1 Rgoue
Level 2 Rogue
Level 3 Ranger
Level 4 Ranger - replace 2nd level ranger benefits that do not advance with an ASI level with the ASI. Benefits that do advance on these levels, such as spellcasting progression, continue to accrue normally.
Level 5 Ranger - apply the remaining level 2 ranger benefits that do not advance on an ASI level for this level, and continue to advance the ones that do advance, such as spells.
Level 6 Ranger - apply the remaining level 3 ranger benefits that do not advance on an ASI level for this level, and continue to advance the ones that do advance, such as spells. We are now in the same place we'd have been under the normal progression rules for a Rogue 2 / Ranger 4 character.
