The main problem is game option. For example, as a Seasons player, I and my friends ( including top players ) always play with "All expansions + promo cards" option. However, in arena mode "Tournament card set" is on. That's quite wired because most players prefer 'All expansions'. I never try arena mode for Seasons in fact.
If Arena Mode aims to clarify players' level then different options must be a obstacle. Different option make game very different in depth and further strategy. For Seasons and RftG, actually, different card set makes totally different game. Thus players would be divided into sub groups, who disliking the arena option will never play arena mode, and others always play in arena mode. Even the options switch in different arena seasons, thing won't be better.
Personally I perfer to make different options tolerable in arena mode, or at least 'selected options'. I wonder if it's technically difficult or not.
No matter what to the end. Big THANKS to the developers