Maribel is an odd one when it comes to heavy armors. Generally they excel in health and defense and attack less so, yet Maribel while still excelling in health, lacks in defense and makes up for it in attack. It is also true that in terms of skill activation higher rarity units are always quicker and Maribel is good in that respect.

It comes down to preference I think to some degree. I personally want a heavy armor that takes hits though having one deal them out is good in some situations. Again to reiterate, Maribel is a good choice when she is fully maxed out and awakened. You can even have her skill be under-powered because it always doubles attack and defense.

The key thing that is preventing me from fawning over her (which I would in other situations) is that she is difficult to get a hold of. Last I checked, drop rates for any event farming are rather atrocious. I have been trying to farm for mages in the last event for nearly its entirety and I only got two, and that was for two weeks where this is for one only.

TLDR: Maribel is a good choice of unit. However she would be difficult for some players to max out and that would take time. Early players should rather focus on other units for either defense or attack and have Maribel be a long time project until she is usable.