My biggest complaint about Buildcraft is not that it lacks energy storage, but rather that some really important parts of the setup (Gates, Facades, and Pipe Plugs) are all locked behind one of the highest tier/late game systems: the assembly table & lasers.
It's really annoying to not be able to build a proper pipe setup with BC stuff, because I can't make any of the above, until after I've already built a large amount of BC systems since Lasers are pretty much the last part of BC I get to. I end up using TE pipes instead, which while easier to use, are FAR more expensive resource-wise for most setups compared to BC pipes.
Plugs are especially egregious in this regard, I have no clue why those must be laser crafted, when they are supposed to be used to make early game items (pipes) work properly, are very inexpensive resource-wise, but completely unavailable til late game. Facades are also a little weird being laser crafted, but less of a problem than plugs. Gates seem fine being made by lasers on the other hand, and also seem like a late-game item worth the investment.
I use both BC & TE both, and I actually like building the laser system, and feel like Gates/Chipsets/AE Chips, are fun to craft using it. I actually wish I could use it to craft Forestry electronic parts as well, some more late-game items to craft with it would be nice instead of things like plugs.
Still I think BC is mostly fine. It's a well written mod, and it's mostly okay that I don't want to use every last bit of it. I just wish a few things were not locked behind such high investment breakpoints, and then I would probably use parts of it more. I would be sad if it moved in a direction to specifically stop working with other mods like TE, but that's something I keep hearing people like the SandGrainOne & CJ mentioning repeatedly.
Edit: Not a complaint, but to be clear, the OP is misleading. RAILCRAFT provides BC storage by your example, not BC by itself. A small distinction, but a real one, as storing BC energy in BC glass tanks would be ... silly in large amounts with BC liquid tanks instead of Railcraft Iron/Steel tanks. Doable still, but certainly you would have to redesign your entire structure around it.