Evaluating vendor reliability can be very nebulous and subjective, but it does not mean you can skip it. This is especially important for web software, since it lives on the vendor's servers. If they go out of business, they might shut down altogether, and you will be rushed to extricate your data and find a new solution in a matter of weeks or even days!
We do not mean to scare you - these situations are quite rare - but you do have to conduct your basic due diligence:
Does the company website look professional, and provide full contact information and a physical address?
Is it clear who are the people behind the company and how big is the team?
How long has it been in business?
How many client installations does it have?
Does it provide you with the tools to extract your key information (e.g. contact and membership records, event registrations, payment transactions) at any time?
Of course, all of this does not mean that you should automatically exclude young companies who just came up with a wonderful new product - but you do want to evaluate them carefully and weigh their reliability with all other aspects.