Despite what some might call significant abuse of their capabilities 153s generally are quite reliable. OK, if they have an engine failure they're much less likely to be able to limp home (but do have the delightful innovation of being able to transfer the toilet header water should they lose coolant) and they're usually easy enough to fix when they do go wrong. For many years the East Midlands service relied on kicking out a 153 when all else failed.
It is important to note that the East Midlands unit fleet has never really been representative of the normal way of working with some off peak spares etc - since 2015 in particular every possible unit has been needed in service and with 30 year old trains that will always affect your availability. You would be lucky to have one 153 spare at Nottingham during the day.
The depots at Eastcroft and Etches Park have worked miracles since 2008 with the dud hand they were issued - a disparate fleet comprising mostly units other TOCs wanted rid of.
Class 156 has issues with the door runners (lots was spent on the ex EMT Porterbrook units to replace the whole door step assembly but that work hasn't been carried out on the Angel 156s or ex GA fleet and it still relies on plastic wheels that get damaged) and they all get a bit hot and bothered when it's warm.
My mileage on the EM 15x fleet is well into 6 figures and I've never required rescue, have only rescued another train once and have had to come out of service in a controlled manner less than 5 times.
Oh, yeah, I will add that I have nothing but respect for the depots who have had to deal with an aging fleet with effectively zero spares and duds from other TOCs.
Especially with the 153 fleet, I remember when I was commuting on the Matlock line how just one of the two booked 153 units would arrive, leading people to be left on the platforms once we left Belper.
Peak time cancellations were a couple of times a year, commuting about four days a week.
Considering many of the areas see an hourly service, the pressure of making sure it runs is pretty immense!
But yeah, it's not the fault of EMT (now EMR), it was the fault of government locally and nationally that ignores the needs of the railways and the people who rely on it.
I hope the 170's will provide a needed boost in capacity, speed and reliability, even if they are aging somewhat too.