In a garbage collection environment it can be an issue with regards to memory management

In typical singleton implementations, once you create the singleton you can never destroy it. This non-destructive nature is sometimes acceptable when the singleton is small. However, if the singleton is massive, then you are unnecessarily using more memory than you should.

This is a bigger issue in languages where you have a garbage collector (like Java, Python, etc) because the garbage collector will always believe that the singleton is necessary. In C++, you can cheat by delete -ing the pointer. However, this opens its own can of worms because it's supposed to be a singleton, but by deleting it, you are making it possible to create a second one.

In most cases, this over-use of memory does not degrade memory performance, but it can be considered the same as a memory leak. With a large singleton, you are wasting memory on your user's computer or device. (You can run into memory fragmentation if you allocate a huge singleton, but this is usually a non-concern).

In a multithreaded environment it can cause bottlenecks and introduce synchronization problems.

If every thread is accessing the same object and you are using a mutex, each thread must wait until another has unlocked the singleton. And if the threads depend greatly upon the singleton, then you will degrade performance to a single-thread environment because a thread spends most of its life waiting.

However, if your application domain allows it, you can create one object for each thread -- this way the thread does not spend time waiting and instead does the work.

Headache from testing prespective.

Notably, a singleton's constructor can only be tested once. You have to create an entirely new test suite in order to test the constructor again. This is fine if your constructor doesn't take any parameters, but once you accept a paremeter you can no longer effective unit teest.

Further, you can't stub out the singleton as effectively and your use of mock objects becomes difficult to use (there are ways around this, but it's more trouble than it's worth). Keep reading for more on this...

(And it leads to a bad design, too!)

Singletons are also a sign of a poor design. Some programmers want to make their database class a singleton. "Our application will never use two databases," they typically think. But, there will come a time when it may make sense to use two databases, or unit testing you will want to use two different SQLite databases. If you used a singleton, you will have to make some serious changes to your application. But if you used regular objects from the start, you can take advantage of OOP to get your task done efficiently and on-time.

Most cases of singleton's are the result of the programmer being lazy. They do not wish to pass around an object (eg, database object) to a bunch of methods, so they create a singleton that each method uses as an implicit parameter. But, this approach bites for the reasons above.

Try to never use a singleton, if you can. Although they may seem like a good approach from the start, it usually always leads to poor design and hard to maintain code down the line.