Upon the massive deployment of Cooperative Intelligent Transportation Systems (C-ITS), services developed for connected vehicles need an efficient, reliable and predictable network access to assure proper operation. European Telecommunications Standards Institute (ETSI) Release 2 services cannot be deployed using a single control channel, and require the management of multiple channels used simultaneously by applications. Because of this, ETSI defined a mechanism called Multi-Channel Operation (MCO). Two simple MCO load allocation mechanisms have been recommended, load balancing and sequential filling, but they have not been evaluated in detail until now. In this paper, these two mechanisms, as well as their congestion control variants, are evaluated. An open-source simulation framework has been implemented for such a work, which is open to future extensions. Then, through multiple evaluations following the ETSI simulation setup, we discuss the behavior of these mechanisms under scenarios with a highly congested medium, employing different traffic loads and vehicular densities. Our results show that MCO improvement is limited under high-load conditions, by saturation of channels before switching to a new one (sequential filling) and synchronization of channel assignment among vehicles (load balancing), and the introduction of a simple reactive congestion control does not improve their performance. The main limitations are examined, and recommendations are provided to guide the evolution of these mechanisms.