They have certainly built some headaches, but inherited a number of them too. In the UK, Vampire and Colossus are high on the list, Nemesis is obviously the first to 'come of age', although you can certainly argue they have had 15 years service out of it in their ownership.
Slammer was a money pit which they eventually gave up on. Enterprise at Alton is in a similar bucket, albeit on a slightly smaller and more mainstream scale.
Further afield, Sequoia, Colossos, Magic Mountain etc have also had their challenges and to their credit, they have responded on a case by case basis as appropriate.
Merlin don't deserve credit on a number of fronts, but if you look across the portfolio, I think rides are generally looked after to a good standard from an engineering perspective, I've never felt unsafe. However, cosmetically that is patchy, to be polite.