How to compare CiviCRM to Salesforce?
Aside from the general arguments against Salesforce, which are well-detailed elsewhere - it's designed for business-to-business interactions.  That results in odd architectural decisions.  For instance, all individuals must have an organization they're tied to.  That often doesn't make sense in the non-profit world, with individual donors, social services clients, etc.

To combat that, a group of folks with the financial backing of the Salesforce Foundation created the Salesforce Non-Profit Starter Pack (NPSP).  This is available at no cost, and alleviates a lot of the pain of using Salesforce in the non-profit context.

Unfortunately...in the past year, Salesforce Foundation has decided to create a new package called "NGO Connect", and it's much more expensive.  Instead of 10 free user seats, you get 2 - and after that it's $90/user/month.  Salesforce Foundation has started to redirect its resources there.  As a result, a lot of features that many non-profits consider essential are ending up in the more expensive NGO Connect, and will likely never end up in the NPSP.

Whether this matters to your clients, who sounds like their CRM needs are quite basic, I'm not sure - but this is the primary concern with the non-profit Salesforce ecosystem right now.

EDIT: By request, I'm updating this with more info.


Here's a comparison of NPSP vs. NGO Connect.  I wouldn't expect features developed for NGO Connect to be available for NPSP users, or they'd kill their differentiation.  In fairness, NPSP seems to still be under active development.
The power of Salesforce is in its integrations.  However, the integrations are a) additional paid packages, b) are generally priced for enterprise use.  You can end up spending a lot to get a strong Salesforce experience.
The Salesforce real-time API is slow, and you're subject to API limits.  Those API limits are dinged every time you use a non-Salesforce native integration.