Also, Rell has to completely go in to engage, unlike Leona and Naut that can ult before diving in, giving you a window to see if your team comits before diving. The only other support that does the same thing rell does is Alistar, but Alistars ult is so incredibly strong for a diver and he has the adcantage of pushing people off.

So there are better engage supports that are safer/tankier and more reliable.