What makes a wall-mounted heat pump genuinely easy to fit?

Compact wallmounted equipment can look easy until the room, exterior wall, condensate route, and outdoor unit location are all considered together. A smaller indoor unit doesnt help much if the only practical location creates a dificult line path or poor service access. Im not looking for installation instructions; im trying to understand which site constraint removes the most otherwise good options before a professional layout is finalized. What would you check first when deciding whether a compact wall mounted system actually fits a space?