Most people will say that Rhino isn’t bad, just too situational.
In pubs, you can absolutely destroy people, even on offense, because they don’t coordinate. Just one person making a callout that “Rhino is going [Location]”, and your team can take him down. (I know because I callout where Rhino is roaming, and his head is such a lovely target.)
In competitive formats, he’s a joke. Coordinated teams will focus him. Rhino might get a kill or two, but if he doesn’t get both medics, everyone is still alive and fighting. There’s not much reason to run Rhino over Fragger.
Rhino is a close range monster. No other weapon can rival the DPS of the minigun, so it can only really be used at close to mid range. The current meta is based around mid to long range engagements, making him even more irrelevant.
The problem in buffing him is twofold. Buff the range on the minigun, and now he’s too versatile. Give him mobility and he can then gapclose, removing the one weakness he has.
I don’t like playing Rhino. I find him too slow in a fast paced game. His playstyle doesn’t encourage good duels, it encourages flanking or camping with close range, high damage weapons.