Rammus rework might have made him a little too niche.

Specifically his W, the rework as a whole I find met most if not all of what Riot wanted to accomplish but the biggest issue currently I personally find is that he's too reliant on items to be tanky now especially when against mixed damage teams. Before if Rammus was behind he could start maxing W for the extra tank stats so that is a flat 120 Armour and 120 MR just for pressing W ,but currently since his W scales off % total resistances he needs items for him to be tanky but if he doesn't have the gold to buy items then as you might have guessed he's useless as a tank. Now one of the biggest issues here is that he no longer is as useful against mixed damaged comps since he would need MR to make use of his total resistances but he doesn't get MR usually before for free anymore.

Let's take a Rammus with 30 armour /12.1 MR in runes, level 7 (52 armour/38 MR), 5 points in W and .

Old W: 40/60/80/100/120 ⇒ Old rammus with W turned on would have 222 armour and 170MR. (No slow)

New W: 20 flat, plus 50/55/60/65/70% of Rammus’s total resistances New Rammus with W turned on would have 159 Armour and 105 MR

Now ofcourse he scales much better with items the later into the game he goes but his inability to get back into the game once gold deprived is a major issue even if only 1 mage is on the enemy team since he needs armour first. This essentially means he is even more so constricted towards being picked against AD only comps.