What do science ships get, moving from T4 to T5, that makes up for the fact that every other type of ship gets an extra weapon and they don't? Do their stats increase to a greater degree than escorts or cruisers? If they don't, then it is a simple fact that T5 science ships are underpowered; regardless of who is flying them.
I should clarify that stuff like sensor analysis, or the innate subsystem targeting, don't count; since they are available to science ships of all levels, and so are not part of the tier system. I suppose, considering those abilities, that it could be argued that they are overpowered until T5, rather than underpowered at T5; but having levelled up several characters in sci ships, I'd find that hard to swallow myself.
To be honest, Sci ships are over powered compared to escorts and cruisers before t5. This tends not to be noticed because it takes some time to learn sci well and by the time you get a handle on it your are t5, but in the early levels sci ships are way out in front. Escorts don't even start to catch up till the first defiant, and even then sci pulls up ahead.
Low level escorts and cruisers are garbage compared to the sci ships. Getting their last round of goodies is what finally puts them in the same realm as sci.
The real problem comes with gear scaling. If we are all in blues everything works out about perfect. As better and better gear comes out average durability and DPS goes up across the board. This increases the value of DPS considerably. The higher DPS and durability rise compared to the content you face, DPS begins to rapidly reduce the incoming damage you take, which in turn increases your durability, and reduces the need for healing. As the situation continues, the gap increases. Higher DPS leads to faster kills leads to less damage and healing, increasing the value of DPS, in a vicious cycle that doesn't end unless the content we face increases in difficulty as we progress.
But... In STO we don't have that. The more recent content as a whole has been easier then the previous content. Fleet and rep events are easier then STFs. The only content we've seen that turns up the heat and starts to add some movement to the other side of the equation is the No Win Scenario and Hive Onslaught. Notice, in both of those 2 places healing and sci abilities start to become useful again? It is due to the increase in dificulty of the encounters. It puts the balance back in the equation.
The only problem with sci ships when the content is balanced vs player ability is the fact that a carrier is going to out perform it every time. The trade off is mobility. I can live with that. Doesn't matter how BA my carrier is, sometimes I feel like actually turning
Edit: One of the other things that puts a carrier up ahead is lack of Sensor Analyses. The carriers get their extra DPS from pets instead of SA. I don't ever have to think twice about breaking off my target to toss a heal or a CC, because I don't lose my SA stack to do so.