You will get tons of different opinions that are very hard to sift through on WH. The reason for that is because everyone adopts a very different point of view and looks extremely subjectively on the performance of a class.
Here's a bit more of an objective viewpoint from someone who played the class, but isn't an active WH :
Yes, WH is in a bad state right now. No, not because the WH as a class is weak at its job. Because a WH has a niche role which is less prominent in the meta game and because several other classes outperform him in most places. The playstyle of a WH is in a bad state - not the WH itself. Also because a WH requires a large amount of dedication to achieve usefulness and there are massively dedicated WHs who you will compete with who had years of a headstart ahead of you.
Point by point on the above:
1. The meta game is large-scale fighting and highly organized small scale fighting. These are not the strong suit of a WH.
2. White Lions and Slayers have 1) aoe and 2) better tools for getting in and out of a fight and dumping damage easier (yes, stealth is theoretically good for getting in and out of a fight, but in practice within scenarios and small scale it is not). WHs have arguably better single target damage, but this is achievable only at high gear levels (more on that below) and will often be much harder to dump. You can outperform a WL/Slayer/shadow warrior on a single target train. But most of the time you won't. Therefore you will not be as desired unless you're extremely well known. Also at the higher meta game where this even matters, those classes will also be overgeared and likely bring stuff to the fight you don't.
3. The ganking/soloing playstyle a WH would normally adopt is in a very bad shape right now. Soloing is highly discouraged, 1v1s are as far away from balanced as possible and this has in turn motivated more and more soloers to leave, meaning 1v1s, stragglers and other things you may wish to hunt as a witch hunter are extremely rare as of late. Even more so outside peak EU times.
^ these points are almost always where the arguments for whether a WH is good or not become confusing. Because the class "Witch Hunter" is pretty ok, but the game Warhammer Online is very bad for witch hunters
4. For a WH to perform properly, you need massive dedication. Mostly gear. The WH auto attack build for example that can drop hilarious amounts of damage is warlord/sov iirc. This in itself is not the problem, it's more that IF people need a WH they will probably only need 1 or 2. And due to the amazing class fantasy and style of the class, weakness or no weakness there are dozens of extremely dedicated WHs that have been playing for years, have perfect gear, perfect skills and lots of reputation. You will be competing with them for group content.
So, to sum up with conclusions, a WH will perform well, you won't feel weak, its weakness is NOT that you can't kill things - you will kill things, a lot, quickly. But you will have trouble finding groups or playing the meta and will likely have to settle for less than most. You will also absolutely need to make friends and endure the hardships of soloing on a server where soloing is extremely discouraged.