To a great extent this will depend on the type of leather you have and what you are trying to clean.
Foam cleaners are best and safest for deep cleaning and there are specialist cleaners for Aniline style leathers and specific problems such as dye transfer, odours, urine etc.
The best results in any cleaning job come from identifying the leather and selecting the best product and process for the job in hand rather than a one product fits all approach.
Always happy to help if you need advice.
Gary I cleaned the odd bit of leather for twenty years. Got my wife on judy's course and we have a fantastic leather business added to my carpet and upholstery business. It's a whole world of oppertunities with good ticket values and higher end clients, susan is always busy. Get trained, no point having a go without.
To a great extent it depends upon the companies that actually put resources into development of product.
Last years fantastic cleaner for example, may be this years, ok product.
My advice would be to try new products at every opportunity.
A manufacturer confident in its product will be happy to send you a small sample.