There are two types of information loss possible: the actual pixel information, and metadata.
First, RAW data is not actually visible (in any meaningful sense). It has to be interpreted to be seen. The reason is because, in a RAW file, each pixel only has information on one color - red, green, or blue. To determine what color the pixel really should be, it has to combine information from nearby pixels as well. This can be done in many different ways, and is inherently an interpretation.
With a TIFF, you lose the actual pixel information. That's because the TIFF does an interpretation and gives you the results, while throwing out the RAW data. That interpretation may be of the highest possible quality. But it can't be undone. And it's always possible that tomorrow there will be a better way to convert RAW files, or one that it better suited to your creative vision. With a TIFF, you've lost that level of control. It may not be a big deal, but it is a loss of information.
With a DNG file, the default is lossless with respect to actual pixel information (although the DNG standard allows you to choose a lossy method of compression). So it doesn't have the shortcoming of a TIFF file. However, it's is still lossy vis-a-vis the original RAW file because of metadata. DNG is a universal file format, and Adobe did its best to keep all the information possible. However, even if all the metadata is in the DNG file (which I assume is true, but I'm not 100% sure), DNG readers don't necessarily know how to use company-specific information -- so it is effectively lost. For example, let's say there is a company-specific feature (I'm thinking Dynamic Range Optimization on my Sony cameras). The DNG may record my camera's setting, but Adobe Camera RAW can't use it to duplicate the effect. Only the company-specific software would be able to do that. But the company-specific software can only read the original file, not a DNG. So, even if all the metadata is "there", it's still effectively lossy.
Of course, DNG has its benefits. As an industry standard, it's likely to have a near-infinite life, so you won't lose the ability to read the files 20 years from now. That's why I used to convert all my RAWs to DNG. On the other hand, it is somewhat lossy. That's why I've recently stopped converting my RAWs to DNG. I wish I could go back in time and undo what I did.
But it's not a very big deal. To me, the more important thing is that all the pixel-level information is there. I haven't used Sony's software in years. To be honest, even TIFFs don't really lose any pixels (i.e., there is no compression); they just lose the ability to benefit from superior conversion in the future.
