Mesh simplification & decimation algorithms

Hello i recently started to tackle the various mesh decimation & simplification algorithms, mostly from Hoppe & Melax. The algorithms in question are the Quadric error metrics mesh simplification & the Progressive mesh simplification. I found a few good github starting points (VTK, hoppe's & melax's githubs etc.) to try and test the various implementations but i run into some problems:

1. I tried to simplify a few of mine and a few of the provided (inside the code base) meshes and i noticed that not all meshes survive the same simplification. For example a 400 vertices robot mesh can be simplified down do 10-20 (or about 3-7% of the src vertices) vertices without any problem no missing faces or huge topology distortion, yet an Eagle mesh with 3000 vertices can be just simplified to 1900-2000 (or about ~60-70% of the src vertices) vertices before i start noticing that the mesh starts to lose faces (which leads to a mesh with quite noticeable number of holes). And that is something i noticed with quite a few models (be it mine or the provided). I would like to know what is the actual cause which stops / prohibits some meshes from being simplified as much as others. I am probably asking a quite stupid or obvious question

2. Even if the simplification was perfect i still have difficulty figuring out how would i preserve the vertex appearance (uv coord, tangents, normals etc.) I read a few papers - one in particular from Hoppe - New Quadric Metric for Simplifying Meshes with Appearance Attributes. But did not find any real world examples or at least snippets of code to give me a better grasp of the technique

PS: Just as a side note VTK is a beast of a library never used it before i find it a bit heavy or maybe bloated for most people. But i really liked it !