Will staying longer than specified on a UK Visit Visa affect a future spouse visa application?

Is there any issues with staying in the UK longer than specified on a UK tourist visit visa as long they go home before the 6 months runs out on the visa?

Yes, there can be. See for example UK visitor visa refused due to previously staying for too long and not having strong ties. How should I reapply? Visas are issued on the basis of an assessment of the information provided in the application. Staying significantly longer than stated may call into doubt an applicant’s truthfulness for example (did they deliberately mention a shorter visit in the hope of improving their chances of success?) and/or ties to home (how is it that the applicant is able to spend months in the UK and does that accord with having a compelling reason to leave?). It’s a good idea for anyone who has stayed much longer than originally stated to explain why in their subsequent visit visa application.

Would staying longer than specified in the visit visa application affect a future spouse visa application at all, or just future visit visas?

The Immigration Rules governing applications from spouses and civil partners https://www.gov.uk/guidance/immigration-rules/immigration-rules-part-8-family-members are a completely different set of eligibility criteria than for a Visitor visa. The applicant is intending to settle in the UK, therefore considerations of length of previous stays as a visitor should not have an impact, providing the Rules relating to Visitor visas were not breached, which might be construed as grounds for refusal https://www.gov.uk/guidance/immigration-rules/immigration-rules-part-9-grounds-for-refusal.