The purpose of this article is to show that there are many differential viscoelastic models for which the global existence of a regular solution is possible. Although the problem of global existence in the classic Oldroyd model is still open, we show that by adding a nonlinear contribution (proposed by R. G. Larson in 1984), it is possible to obtain more regular and global solutions, regardless of the size of the data (in the two-dimensional and periodic case). Similarly, more complex appearance models such as those related to “pom-pom” polymers are interesting and mathematically richer: some “natural” bounds on the stress make it possible to obtain global results. On the other hand, in the last part, we show that other models clearly do not seem to fit into this framework and do not even seem to have a global solution in time. These kinds of results allow us to highlight the advantages and disadvantages of such viscoelastic fluid models. They can thus help rheologists and numericists to make choices with new arguments.