These blanket statements are not helpful to novices. There isn't anything "bad" about splitters. Some are better than others.In-band splitters are fundamentally bad and should be avoided.
The points against splitters are twofold. First there can be attenuation of the transmitted or received signals. This is usually specified in the technical data of the splitter. It is possible to build a splitter with very low attenuation. Second, is the time sharing of transmission as the VHF and AIS cannot transmit at the same time. This second issue is not a big problem for most cruising boats.
There are also points in favor of splitters. Ease and speed of installation are a good thing. Usually the mast head antenna can be used which due to height can more than make up for any splitter loss.
So maybe we should just list the pros and cons and let the reader decide what is "good" or "bad" for themselves.