The answer to your query is dependent upon how you define problem. Ipsos, as an example, is a well established international corp; the chances of them going "belly up" is very remote. The reasons I decided to end the relationship were a work:pay ratio that was completely unacceptable and a lack of communication on their end. Therefore, I had problems, but perhaps their low pay and silence would not be a hindrance for you.
I have been contracted with the following MSCs for many years and have never experienced the slightest problem with being paid.
1-Intelli-shop
2-Service Sleuth
3-Mintel
4-Informa
5-Pinnacle
6-Quest 4 Best
The above are companies that immediately come to mind, but none will pay you this week or next. If quick payment is a hot button for you, then be prepared to be paid a much lower fee for your work. In addition, even with the above six, I carefully weigh the work:pay ratio. The old adage that all that glitters not being gold applies to shopping.