- You can install dpkg from AUR which will provide a probability to install .deb packages if needed
- You have no guarantee that there is an equivalent for every Ubuntu package, or that it will work correctly after you install it with dpkg.
I, personally, switched from Ubuntu to Fedora and than to Arch, and never had a problem with availability of some packages available in Ubuntu on Arch. Anyway, even if you can't find exact package, you can always build it from sources and in my opinion in Arch this process is somewhat easier. Arch nonetheless have another problem, fast packages update prevailing over correctness and consistency of packages, so you have no guarantee that after upgrade your system will work correctly (however, I faced this problem only once during 3 years of Arch experience)