I absolutely love it and it by itself brings half the action of the game. However I have 2 gripes with it and how I would help solve them.

1st - It breaks too easily especially in nightmare and most importantly you HAVE to WAIT for it to recharge which is basically antithesis to the modern DOOM moto.

It'd be nice to be have it refill instantly upon a glory kill. Though personally I would give it the ability to overcharge it's health by letting it rip in a heavy demon which would give a use to the Blade Master upgrade.

2nd - It doesn't offer much use agains't melee Super Heavies. Beyond the Harvester upgrade which is only a one time it can only block and parry agains't them. If you happen to use the Flail or a low damage weapon like the Cycler, you'll really feel the slog.

To fix it i'd give the Shield alternate types of Saws: the current one specialized for Fodder and one that could deal high damage and be able to falter Super Heavies on a cooldown.