Bloodletters - not to be brought in with DS. The fact that they sit in a clump for a round of shooting meant that they did nothing other than bring the soul grinder with them via the instrument of chaos and then got blown to bits my blast weapon wielding tomb blades.
The DS is pretty useful when you have to claim a Maelstrom objective or threaten a lone backfield unit, but... yeah, they're fragile. Do bear in mind you can Run to spread out the turn they DS in for some blast protection, but... much dedicated shooting aimed at them and they'll go poof.
Berzerkers - they put in work, certainly, but are so expensive compared to the bloodletters that i don't think i'll use them anymore. They are a like a top shelf hooker...they are totally fun to use and you like having them around but at the end of the day you can get 90% of the experience for way less money.
Land Raider - since this isn't a part of the war machines formation you have to bring a whole CAD or go unbound to bring it in...that annoys me.
A CAD at least allows for some ObSec, so you can lock down some objectives. Plus, hey, Rhinos make for cheap Blood Points and transports in the FA slot, just not being Assault (which is the clincher).
That the Bloodletters are only I4 hurts them, I think, for how fragile they are. A lot of Eldar/DE/Wraiths and stuff will make them poof before they get to strike--but against 3+ armor I4 or lower opponents, they certainly do work. When not assaulting into cover with no assault grenades.
In the Zerkers vs Letters, the much higher cost of Zerkers/potential Blood Point is offset by buying the Zerkers a Rhino or something, while the Bloodletters... well, buying them a Land Raider is an expensive ride.
Soul Grinders ruin folks' days, certainly. I wish there was some way to compensate for its BS3 & WS3 without, say, bringing a Lord of Change for Prescience in the area or something, but...