Yes, they are. The time to buy a 1992 190E was sometime around the late 90s or early noughties, and with care it would have been a good car for maybe another 10 years or so before it started getting troublesome and handing in big bills. For 10k miles a year it would have been great and would have needed only minimal repairs. However once it got beyond 15 years/150k miles the faults and wear would have become apparent and the snag is that they start to gang up on you. So you get to the stage where the thing is consuming a bit of oil. OK, we can live with that. Eventually I'll have to repair it but it will last a year or two like this. Then the gearbox starts to wear, nothing much at first but 2nd and 3rd are getting a bit baulky, especially with a cold engine. No trouble, just double it into gear when cold. The PAS leaks a bit, but you can keep that topped up. The aircon packed up a while ago. The back axle whines a bit under acceleration. Well, it's an old car. You can live with this. It's an old car and it feels a bit that way, but there you are.
Then it's MoT time. Hmm, says the man, it needs a new brake caliper and that's trashed the disc. There's a bit of routine bush wear and the odd bit of suspension needs doing. Say a broken spring or leaky shocker. You're doing 10 k miles a year after all. But worst of all the brake lines are corroded. It's a 15 year old car, these things happen. Sorry mate, but this is a lot of work, about £800. Maybe £1k.
What are you going to do? Sort out the MoT, knowing that the engine is worn, the gearbox likewise, the back axle whines, there's no aircon and you have to top up the PAS every fortnight? Or are you going to bite the bullet and get an engine rebuild, gearbox, axle, PAS repair? £4-5k on a car worth, what? Less than that. You're in a bind. You spend £1k on a car that isn't nice to drive and that might come back and bite you or you spend more than the thing is worth on making it nice. Or you get rid.
Old cars, especially old luxury cars, are like some women I knew in my younger days. They were fun but you needed to know that it wasn't for ever and to have an exit strategy to deploy when necessary and know where the fire exits were in case of the need for a hasty exit.
Take a standard block and manufacture a new twin cam head with custom pistons - it is a recipe that has worked for them time and again.