This is a feature, not a bug. Unlike most other games, spending money in E&P does not guarantee you a powerful team. Of course you’d expect to do better the more money you spent, but there is a chance you’ll be unlucky and flush your money.
Similarly, F2P players can get lucky, and grinders can grind their way to a good team. Being a big spender gives you a better chance of developing a powerful team, but it does not guarantee it.
This is one of the best aspects of the game, I think. Certainly it’s what has kept me playing for more than a year, F2P, and I have heard similar things from other F2P players: in E&P we can stay competitive. I suspect it’s also one of the things that makes the game profitable - those who can be tempted to risk money summoning heroes can probably be tempted to risk a little more if the first summons fails. And a little more. And a little more.
I’d be entirely behind E&P revealing the chances of success for each pull. It seems just a tad shady for them to keep the underlying odds secret - it leans a little towards the unethical and exploitative. But if what really rots your jocks is the fact that one can’t simply buy success in this game, then I have no sympathy for you.
There are plenty of games in which you can just buy success - go play them. This game has earned millions of players (and millions in profits) by departing from that paradigm and making players rely on the goodwill of the Gods of Randomness. Sometimes they love you and sometimes they hate you. If you can’t handle that then E&P is not for you.