If we are talking resi, clearance issues on new construction arise when 'who is there first'. When the EC is doing trim out for finals, he mounts the discos at the location. The CUs are dropped after, and sometimes the disco is behind them, or the tall units block the disco.
When the AC line set is in & charged, guess who has to move what??
Replacement CUs usually are OK. Best comment I can add is each job has to be treated individually.
Commercial RTUs are usually no issue.
Do we live in a perfect world, No.