Sometimes booking with a small tour operator can be the better choice. But people often think that the larger the company, the safer they are. Experience has shown that this simply isn’t the case. Just to give you a recent example:
Lanta Tours is a large tour operator. They just went broke, and now there are hundreds of tourists who have already paid for their trips, but are now stranded in Thailand. Why? Because Lanta Tours didn’t pay the hotel bookings, and not even the return-tickets of their customers. So now the Tourism Authority of Thailand (TAT) and the Russian embassy are doing what they can to find a solution for these people – most importantly accomodation and a flight back to their homecountries.
Because the hotels have cancelled their bookings and kicked the tourists out (which is understandable, since they’ll not get any money and it’s not their fault that the tour operator went bancrupt).
These things happen again and again, and one reason is that every time a large company like this goes belly up, there’s a lot of money that gets lost, but there’s also a couple of people who usually stuff their pockets first and then disappear.
When you book with independent Thailand tour guides or small tour operators, there’s usually a much higher reliability, simply because you’re interacting with the people in charge of taking care of you, rather with some operator on a payroll.