I have ground up several ships to test the system out and get seed money, here are my thoughts. Some of this was done pre-knowledge tree removal, or I would never have ground the brig and cerb.
7th Rates are in a good place, anyone can grab one and become proficient in a day with some missioning. Not much to say here.
6th rates are also pretty good, it doesn't take forever to get the first 3 slots unlocked, and the others can be done with some small dedication.
5th rates are not so good. The cerb, frig, surprise, and pirate frigate all took me forever to get to two slots, and I did not bother with three on most of them. Pirate frig I have 4 since I love that ship <3. The rate at which you can earn xp vs how much it takes is rather broken, especially for the heavy frigates. Requirements should be reduced on these since this should be a very common ship class.
4th rates do pretty well, I can get Agamemnon up to two slots without too much work, but more takes a while. However the xp earn rate vs requirements are not broken like the 5th rates.
3rd rates are hurting. The missions they can do yield 650-1000 xp per, with a 46k requirement for the third slot on Bellona. Even if each mission + sailing was 30 min, that is 23 hours MINIMUM to get it partially unlocked; this is a bit much, perhaps you should be able to grind the 5th slot in 20hrs total?
2nd rates I have not done.
1st rates I looked at the requirements and decided not to bother. I have a clanmate who has all the slots, and it took him days and days and days of hard grinding fleet missions solo.
We need to talk about the philosophy of upgrade slots vs how hard they are to grind. Devs have added tons of cool ship knowledge upgrades, and it saddens me that few get to use them; not to mention we can't test them very well without good access to them. Do you need knowledge slots to be successful? No. But they are good rewards for dedicated players, and I think the payoff should come sooner than it does now.
I think all would agree and say that having 1-2 upgrade slots allows you to fight and grind much faster/more efficiently, so why not give the first slot away free on each ship? As it is, you feel like you are just getting started finally unlocking that first slot. The first three slots should be easier in general, but later ones are OK being harder.
tl;dr 5th rates are hurting and need lower requirements, PVP should be given far more XP than it does now. Perhaps 1st slot should be free on all ships, a la green ships pre-patch. It should be easier overall to grind slots 1-3 so people can create builds they like without working their mouse to the bone.
As a minor note, the limited ship slots make it much harder to grind up a pvp ship, since you will want a tough one for grinding and a different one for OW fights or PBs.