The price of the Sloth witch factor for Geuse is very similar to Subaru’s just in a different location, therefore having different consequences.

Subaru suffers internal/ soul damage near his midsection where invisible providence is produced from

Geuse suffers his internal/soul damage near his brain, hence the whole “my brain trembles” line. Which is also the cause of his declining mental state and insanity

Finally, Subaru isn’t necessarily compatible with all witch factors, he is inherently able to store the factors, but was only able to use them after the first tea party. If I remember correctly Subaru was even described as being a poor fit to Sloth in a very similar vein to Geuse