Basically Summer 2019 saw a right old ACMI mish-mash turning up for TUI's flights (and we will see the same for S20) but they got the program covered without too much hoo-hah.
Titan are an established ACMI airline and G- reg'd.
Norwegian are OK, as is Evelop, Air Europa, and Alba Star.
I would be perhaps less than comfy on say Sunwing/Smartwings/avion/freebird/smartlynx/Olympus/ASL/Correndon et al mainly in terms of the reliability of the service and of the comfort, rather than any safety issues.
However, Smartwings have had a huge incident this summer with an engine failure and shutdown on a 737-800NG out of Chania that continued on to home base at Prague.
The aircraft was enroute at FL360 when the #1 engine failed. The crew reported a "maintenance issue" to ATC and requested to descend to FL240. No PAN or emergency call was issued. The crew attempted to restart the engine twice unsuccessfully. The captain decided the nearest suitable aerodrome to be Prague and continued to Prague without informing ATC along the 2:20 hours route after the engine failure until in contact with Prague ATC, when the crew declared a PAN and reported #1 engine had failed. The aircraft landed OK. A failed fuel pump caused the engine to starve, metallic chips were found in the fuel filter. Both parts were shipped for expert examination. An investigation into the occurrence rated a serious incident is continuing.
Previously Sunwing had a major incident taking off out of Belfast.