The types allowed (steel, dragon and fairy) are rather niche. Also, it's a narrow selection. These aren't the major types to appear during regular great league battles to begin with (only one of these in a group - occasionally). There are almost no common good or even decent enough pokemons that are achieveable in these types. Only top-tier, like Tinkatink, Melmental, Azumaril, Altaria - which require many 100s of candies of rare pokemons, 100,000s of star dust, and sometimes 100s of XLs and legacy moves. Don't have these? Then you don't stand a chance.
This is only my second season (but I'm in rank 21 and decent), and I have no worthy qualifying pokemons. Trying to use what I have failed. I tried UL for the first time. I only have 3 pokemons, that are not even good, but could yield mostly 1, occasionally 2 wins per rank. But I don't enjoy it. I don't invest in second attacks for a week, I don't know my pokemons or the opponents', and don't enjoy working with them.
They should have allowed at least one or two major types to participate. It's narrow, unbalanced, and unfair. Now I'm stuck an entire week without being able to play - and many are in the same position. Great league is the only accessible league. And I'm hugely into it, play the maximum every day and wait for midnight every day to play more.