My personal belief is the opposite: STO suffers because its Classes are too loosely defined, and thus each ship has no major purpose in the game and DPS ends up becoming king. When you don't need a tank or buffer/debuffer in encounters there's no reason to not go DPS. So most people end up in Escorts due to expediency.
STO is about my Liberated Borg Federation Captain with his Breen 1st Officer, Jem'Hadar Tactical Officer, Liberated Borg Engineering Officer, Android Ops Officer, Photonic Science Officer, Gorn Science Officer, and Reman Medical Officer jumping into their Jem'Hadar Carrier and flying off to do missions for the new Romulan Empire. But for some players allowing a T5 Connie to be used breaks the canon in the game.
Escort, Science and Cruiser classifications. Are they a problem for STO?
I dont think its a problem for STO except that it limits future playability. IE, ships with too many consoles of a specific type make the ship unattractive to captains of another type, or have weapons that cannot be fully utilized by captains of another type, simply means people have fewer viable options after they have run out of obvious choices. How much of a problem is this? It would be nice if ships were more flexible but I dont think it matters that much really--there are so many other problems that work against long-term game-play, and this is pretty low on the list.
Something kind of tangential, is that ships should be balanced around mass so that inertia, turn-rate, crew size, power core, and so on are scaled in a transparent, consistent manner. But it is also reasonable to give them some functionality based on the ship's "design objective", such as a research ship having fewer weapon hard-points and more power for auxiliary systems, or a warship having the opposite, and so forth. You can also make arguments that things like crew size, hull strength, shield strength, and some other things should be in the class-specific feature list. But for the most part, small ships should move better than large ships, have smaller power cores, etc. Right now the ships are way past that mark, large escorts that weigh more than a small cruiser but still manage to turn faster, science ships being somewhere else off the grid, and so forth. There's no logical demarc that I can find for cannons and beams either (maybe one exists, but I cant find it), which is like saying oh some ships can use mines/torps others cant based on arbitrary decision. Fixing this would go a long way towards making other classes more attractive, right now a tact captain flying a beam cruiser might as well have a KICK ME IM STUPID sign, and other than a few oddballs it will not happen. So balancing the ships around mass would go a long way towards making the ships attractive to other classes, there can still be some flavor but the ships should not be so heavily oriented towards an intended class.
What about Tiers? A ship must belong to X Tier, because of some arbitrary idea?
Ship tiers dont bother me, although I would like to see the tiers realigned to rank. Frigates as tier-1, destroyers as tier-2, escorts as tier-3 and -4, cruisers as tier-4 and -5, etc. It would also make sense that an ~ensisn rank captain would basically be doing patrol missions around Sirius, rather than running long-range operations against intergalactic enemies of the Federation. This would resolve some of the other absurdities in the gameplay. But its waaay too late for that change.