I can think of many airlines that have had multi-day delays over the years. At least they are not taking the easy option of mass cancellations.
TUI is a more complex beast than the other airlines you mentioned. Long haul operations and more small base (single a/c in some cases) operations. Are they having a tough time - yes. Is it more than bad luck, who knows but lets keep a sense of perspective. The vast majority of TUI pax are getting away on time.